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The Spectral Energy Distribution of Dust Emission in the Edge-on Spiral Galaxy NGC 4631 as Seen with Spitzer and the James Clerk Maxwell Telescope
Abstract
We explore variations in dust emission within the edge-on Sd spiral galaxy NGC 4631 using 3.6-160 μm Spitzer Space Telescope data and 450-850 μm JCMT data with the goals of understanding the relation between PAHs and dust emission, studying the variations in the colors of the dust emission, and searching for possible excess submillimeter emission compared to what is expected from dust models extrapolated from far-infrared wavelengths. The 8 μm PAH emission correlates best with 24 μm hot dust emission on 1.7 kpc scales, but the relation breaks down on 650 pc scales, possibly because of differences in the mean free paths between photons that excite the PAHs and photons that heat the dust and possibly because the PAHs are destroyed by the hard radiation fields within some star formation regions. The ratio of 8 μm PAH emission to 160 μm cool dust emission appears to vary as a function of radius. The 70 μm/160 μm and 160 μm/450 μm flux density ratios are remarkably constant even though the surface brightnesses vary by factors of 25, which suggests that the emission is from dust heated by a nearly uniform radiation field. Globally, we find an excess of 850-1230 μm emission relative to what would be predicted by dust models. The 850 μm excess is highest in regions with low 160 μm surface brightnesses, although the magnitude depends on the model fit to the data. We rule out variable emissivity functions or ~4 K dust as the possible origins of this 850 μm emission, but we do discuss the other possible mechanisms that could produce the emission.
... An excess of emission at wavelengths ≥500 μm with respect to the MBB model has been observed in numerous dwarf galaxies (e.g. Galametz et al. 2011;Rémy-Ruyer et al. 2013, in latetype galaxies (Dumke, Krause & Wielebinski 2004;Bendo et al. 2006;Galametz et al. 2009), in the Magellanic Clouds Israel et al. 2010), and in M33 (Hermelo et al. 2016;Relaño et al. 2018). The origin of this 'sub-mm' excess is still an open question. ...
... (ii) BMBB: When fitting the FIR SED with an SMBB model, some galaxies show an excess in the flux at wavelengths ≥500 μm, called 'sub-millimetre' excess (Lisenfeld et al. 2002;Galliano et al. 2003;Dumke et al. 2004;Bendo et al. 2006;Galametz et al. 2009;Bot et al. 2010;Israel et al. 2010;Hermelo et al. 2016). The BMBB model assumes that the sub-mm excess is due to variations in the wavelength dependence of the dust emissivity law. ...
... In particular, we are interested in galaxies that show a so-called sub-mm excess. An excess at sub-mm wavelength has been observed in dwarf galaxies (Lisenfeld et al. 2002;Galliano et al. 2003), in late-type galaxies (Dumke et al. 2004;Bendo et al. 2006;Galametz et al. 2009), and in the Magellanic Clouds Israel et al. 2010). The most significant excesses cannot be explained by contribution from synchrotron, Table 6. ...
jats:title>Abstract We study the dust properties of 192 nearby galaxies from the JINGLE survey using photometric data in the 22-850$\mu$m range. We derive the total dust mass, temperature T and emissivity index β of the galaxies through the fitting of their spectral energy distribution (SED) using a single modified black-body model (SMBB). We apply a hierarchical Bayesian approach that reduces the known degeneracy between T and β. Applying the hierarchical approach, the strength of the T-β anti-correlation is reduced from a Pearson correlation coefficient R = −0.79 to R = −0.52. For the JINGLE galaxies we measure dust temperatures in the range 17 − 30 K and dust emissivity indices β in the range 0.6 − 2.2. We compare the SMBB model with the broken emissivity modified black-body (BMBB) and the two modified black-bodies (TMBB) models. The results derived with the SMBB and TMBB are in good agreement, thus applying the SMBB, which comes with fewer free parameters, does not penalize the measurement of the cold dust properties in the JINGLE sample. We investigate the relation between T and β and other global galaxy properties in the JINGLE and Herschel Reference Survey (HRS) sample. We find that β correlates with the stellar mass surface density (R = 0.62) and anti-correlates with the HI mass fraction (MHI/M*, R = −0.65), whereas the dust temperature correlates strongly with the SFR normalized by the dust mass (R = 0.73). These relations can be used to estimate T and β in galaxies with insufficient photometric data available to measure them directly through SED fitting.</jats:p
... In star-forming (SF) regions, PAH emission appeared suppressed relative to other star formation tracers, while in diffuse regions, PAH emission appeared high relative to the other tracers (Calzetti et al. 2005;Prescott et al. 2007;Bendo et al. 2008). Shell-like features in PAH emission were identified around both galactic and extragalactic SF regions (Helou et al. 2004;Bendo et al. 2006;Povich et al. 2007;Kirsanova et al. 2008;Watson et al. 2008;Dewangan & Ojha 2013), illustrating that PAH emission is suppressed within the centres of photodissociation regions. Additionally, PAH emission was found to decline relative to 24 μm hot dust continuum emission in low-metallicity regions (Engelbracht et al. 2005Madden et al. 2006;Calzetti et al. 2007;Gordon et al. 2008). ...
... To reconcile these two phenomena, we suggest that the PAHs tend to be associated with warm dust (dust cooler than the dust at the centres of these regions but hotter than dust in the diffuse ISM) and molecular gas within shells around these SF regions. This would be consistent with observations of PAHs in shells around SF regions within the Milky Way (Povich et al. 2007;Kirsanova et al. 2008;Watson et al. 2008;Dewangan & Ojha 2013) and in other galaxies (Helou et al. 2004;Bendo et al. 2006). The radiation fields within the centres of the SF regions are potentially strong and hard enough to destroy PAHs. ...
... In another five galaxies, PAH excitation was enhanced around SF regions only within specific ranges of galactocentric radii. Prior results show that PAH emission appear suppressed relative to other star formation tracers within SF regions (Calzetti et al. 2005;Prescott et al. 2007;Bendo et al. 2008) and that PAHs are typically seen in shell-like structures around SF regions (Helou et al. 2004;Bendo et al. 2006;Povich et al. 2007;Kirsanova et al. 2008;Watson et al. 2008;Dewangan & Ojha 2013). Our results along with these other observations suggest that the PAH are most strongly excited in dusty shells around SF regions that can be penetrated by soft UV photons from the SF regions but that are shielded from harder photons that either destroy the PAHs or otherwise inhibit the PAH emission. ...
We have examined polycyclic aromatic hydrocarbon (PAH) excitation in a sample of 25 nearby face-on spiral galaxies using the ratio of mid-infrared PAH emission to dust mass. Within 11 of the galaxies, we found that the PAH excitation was straightforwardly linked to ultraviolet (UV) or mid-infrared star formation tracers, which, along with other results studying the relation of PAH emission to star formation, indicates that the PAHs are most strongly excited in dusty shells around the star-forming (SF) regions. Within another five galaxies, the PAH emission is enhanced around SF regions only at specific galactocentric radii. In six more galaxies, PAH excitation is more strongly correlated with the evolved stellar populations as traced by 3.6 μm emission. The results for the remaining three galaxies were ambiguous. The radial gradients of the PAH/dust ratios were generally not linked to log(O/H) gradients except when the log(O/H) gradients were relatively steep. Galaxies in which PAHs were excited by evolved stars had relatively high far-UV to mid-infrared ratios, implying that variations in the link between PAH excitation and different stellar populations are connected to changes in dust attenuation within galaxies. Alternately, differences in morphology could make it more likely that PAHs are excited by evolved stars, as five of the six galaxies where this occurs are late-type flocculent spiral galaxies. These heterogeneous results demonstrate the complexity of describing PAH excitation and have broad implications for using PAH emission as a star formation tracer as well as for modelling dust emission and radiative transfer.
... This is ∼5× lower than the SFRs from our ALMA and infrared data. PAH emission is generally expected to be suppressed relative to hot dust emission and other commonly used star formation tracers, most likely because the PAHs are more easily destroyed by the hard ultraviolet radiation fields in star-forming regions (e.g., Helou et al. 2004;Calzetti et al. 2005Calzetti et al. , 2007Bendo et al. 2006Bendo et al. , 2008Povich et al. 2007;Lebouteiller et al. 2007;Prescott et al. 2007;Jones et al. 2015;Egorov et al. 2023). Even though the PAH 11.3 m feature specifically was expected to be a more robust star formation tracer than other PAH lines (Diamond-Stanic & Rieke 2010), it works poorly as an SFR metric in the centre of NGC 1808. ...
... The 1.5 GHz SFR is probably low because synchrotron radiation produced by cosmic rays should generally appear broader than the star-forming regions themselves (Murphy et al. 2006). The PAH SFR is low most likely because PAH emission is generally expected to be suppressed relative to other star formation tracers in strong starbursts (Helou et al. 2004;Calzetti et al. 2005Calzetti et al. , 2007Bendo et al. 2006Bendo et al. , 2008Prescott et al. 2007). The extinction-corrected H SFR is low possibly because the central region is heavily dust obscured and because the equations that are commonly used are not designed for such extremely obscured regions (Kennicutt et al. 2009). ...
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of 85.69 and 99.02 GHz continuum emission and H42$\alpha$ and H40$\alpha$ lines emission from the central 1~kpc of NGC 1808. These forms of emission are tracers of photoionizing stars but unaffected by dust obscuration that we use to test the applicability of other commonly star formation metrics. An analysis of the spectral energy distributions shows that free-free emission contributes about 60 to 90 per cent of the continuum emission in the 85-100 GHz frequency range, dependent on the region. The star formation rate (SFR) derived from the ALMA free-free emission is $3.1\pm0.3$~M$_\odot$~yr$^{-1}$. This is comparable to the SFRs measured from the infrared emission, mainly because most of the bolometric energy from the heavily obscured region is emitted as infrared emission. The radio 1.5~GHz emission yields a SFR 25 per cent lower than the ALMA value, probably because of the diffusion of the electrons producing the synchrotron emission beyond the star-forming regions. The SFRs measured from the extinction-corrected H$\alpha$ line emission are about 40 to 65 per cent of the SFR derived from the ALMA data, likely because this metric was not calibrated for high extinction regions. Some SFRs based on extinction-corrected ultraviolet emission are similar to those from ALMA and infrared data, but given that the ultraviolet terms in the extinction correction equations are very small, these metrics seem inappropriate to apply to this dusty starburst.
... PAH emission is generally expected to be suppressed relative to hot dust emission and other commonly used star formation tracers, most likely because the PAHs are more easily destroyed by the hard ultraviolet radiation fields in star-forming regions (e.g. Helou et al. 2004 ;Calzetti et al. 2005 ;Bendo et al. 2006Bendo et al. , 2008Calzetti et al. 2007 ;Lebouteiller et al. 2007 ;Povich et al. 2007 ;Prescott et al. 2007 ;Jones et al. 2015 ;Egorov et al. 2023 ). Even though the PAH 11.3-μm feature specifically was expected to be a more robust star formation tracer than other PAH lines (Diamond-Stanic & Rieke 2010 ), it works poorly as an SFR metric in the centre of NGC 1808. ...
... The 1.5-GHz SFR is probably low because synchrotron radiation produced by cosmic rays should generally appear broader than the star-forming regions themselves (Murphy et al. 2006 ). The PAH SFR is low most likely because PAH emission is generally expected to be suppressed relative to other star formation tracers in strong starbursts (Helou et al. 2004 ;Calzetti et al. 2005Calzetti et al. , 2007Bendo et al. 2006Bendo et al. , 2008Prescott et al. 2007 ). The extinction-corrected H α SFR is low possibly because the central region is heavily dust obscured and because the equations that are commonly used are not designed for such extremely obscured regions (Kennicutt et al. 2009 ). ...
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of 85.69 and 99.02 GHz continuum emission and H42α and H40α lines emission from the central 1 kpc of NGC 1808. These forms of emission are tracers of photoionizing stars but unaffected by dust obscuration that we use to test the applicability of other commonly star formation metrics. An analysis of the spectral energy distributions shows that free-free emission contributes about 60 to 90 per cent of the continuum emission in the 85-100 GHz frequency range, dependent on the region. The star formation rate (SFR) derived from the ALMA free-free emission is 3.1 ± 0.3 M⊙ yr−1. This is comparable to the SFRs measured from the infrared emission, mainly because most of the bolometric energy from the heavily obscured region is emitted as infrared emission. The radio 1.5 GHz emission yields a SFR 25 per cent lower than the ALMA value, probably because of the diffusion of the electrons producing the synchrotron emission beyond the star-forming regions. The SFRs measured from the extinction-corrected Hα line emission are about 40 to 65 per cent of the SFR derived from the ALMA data, likely because this metric was not calibrated for high extinction regions. Some SFRs based on extinction-corrected ultraviolet emission are similar to those from ALMA and infrared data, but given that the ultraviolet terms in the extinction correction equations are very small, these metrics seem inappropriate to apply to this dusty starburst.
... The Hershel PACS (Photodetector Array Camera and Spectrometer; Poglitsch et al. 2010) observed all 61 galaxies in the KINGFISH sample at 70, 100 and 160 μm with a fourfold improvement in spatial resolution over Spitzer MIPS at similar wavelengths. This allows us to trace the increase in contribution from diffuse dust emission (cirrus) with increasing wavelength (Bendo et al. 2006) at an excellent spatial resolution for both NGC 3184 and NGC 7793. ...
... Since we are using the 8 μm as a non-stellar PAH emission tracer, we subtract contribution from the stellar continuum using the SINGS 3.6 μm data following the procedure described in Helou et al. (2004). This removes the starlight contribution to the Spitzer 8 μm band but in regions of weak or low PAH emission, there might still be a significant contribution from non-stellar hot dust emission (Bendo et al. 2006). The 70, 100, and 160 μm data have been taken from the second delivery (DR2) of the high-level data products of KINGFISH in units of Jy/pixel. ...
The study of gas-dust interactions occurring in the interstellar medium of a galaxy is essential for understanding various physical processes taking place within it. A comparison of such events at different locations corresponding to diverse astrophysical environments provides more insight into the star-formation as well as dust destruction conditions and timescales. We present a case study for two galaxies: NGC 3184 and NGC 7793, which are typical examples of a 'grand design spiral' and a 'flocculent spiral', respectively. We investigate the gas-dust correlations at various spatially resolved locations within each galaxy, including spiral arms, using archival data. Moreover, we have segregated the neutral gas into wide (warm) and narrow (cold) velocity components to check the correlations with individual dust emission bands. We find a positive correlation between the gas and the dust, with the total atomic gas emission mainly dominated by its warm component in both the galaxies. We also find the dust population in NGC 7793 to have a greater fraction of emission coming from cold and diffuse, larger-sized dust particles as compared to NGC 3184. This nearby galaxy pilot study could serve as a template for similar studies of larger galaxy samples with analogous morphologies.
... The Hershel PACS (Photodetector Array Camera and Spectrometer; Poglitsch et al. (2010)) observed all 61 galaxies in the KINGFISH sample at 70, 100 and 160 µm with a fourfold improvement in spatial resolution over Spitzer MIPS at similar wavelengths. This allows us to trace the increase in contribution from diffuse dust emission (cirrus) with increasing wavelength (Bendo et al. 2006) at an excellent spatial resolution for both NGC 3184 and NGC 7793. Braine et al. (1993) had observed both the emissions for a sample of 81 galaxies and found a typical line ratio of CO (J=2→1/1→0) of 0.89±0.06, ...
... Since we are using the 8 µm as a non-stellar PAH emission tracer, we subtract contribution from the stellar continuum using the SINGS 3.6 µm data following the procedure described in Helou et al. (2004). This removes the starlight contribution to the Spitzer 8 µm band but in regions of weak or low PAH emission, there might still be a significant contribution from non-stellar hot dust emission (Bendo et al. 2006). The 70 µm, 100 µm and 160 µm data have been taken from the second delivery (DR2) of the high-level data products of KINGFISH in units of Jy/pixel. ...
The study of gas-dust interactions occurring in the interstellar medium of a galaxy is essential for understanding various physical processes taking place within it. A comparison of such events at different locations corresponding to diverse astrophysical environments provides more insight into the star-formation as well as dust destruction conditions and time-scales. We present a case study for two galaxies: NGC 3184 and NGC 7793, which are typical examples of a `grand design spiral' and a `flocculent spiral', respectively. We investigate the gas-dust correlations at various spatially resolved locations within each galaxy, including spiral arms, using archival data. Moreover, we have segregated the neutral gas into wide (warm) and narrow (cold) velocity components to check the correlations with individual dust emission bands. We find a positive correlation between the gas and the dust, with the total atomic gas emission mainly dominated by its warm component in both the galaxies. We also find the dust population in NGC 7793 to have a greater fraction of emission coming from cold and diffuse, larger-sized dust particles as compared to NGC 3184. This nearby galaxy pilot study could serve as a template for similar studies of larger galaxy samples with analogous morphologies.
... Due to the tight spatial connection between PAH emission and active star-forming regions in galaxies, PAHs have been widely used as SFR tracers in the past (e.g. Helou et al. 2004;Peeters et al. 2004;Bendo et al. 2006;Calzetti et al. 2007;Draine & Li 2007;Smith et al. 2007;Kennicutt et al. 2009;Shipley et al. 2016). In particular, the brightest PAH emission feature at about 8 (or 7.7) µm has been used out to high redshift (e.g. ...
We present maps of ionized gas (traced by Pa$\alpha$ and Br$\alpha$) and 3.3 $\mu$m Polycyclic Aromatic Hydrocarbon (PAH) emission in the nearby spiral galaxy NGC 628, derived from new JWST/NIRCam data from the FEAST survey. With this data, we investigate and calibrate the relation between 3.3 $\mu$m PAH emission and star formation rate (SFR) in and around emerging young star clusters (eYSCs) on a scale of ${\sim}40$ pc. We find a tight (correlation coefficient $\rho$${\sim}$0.9) sub-linear (power-law exponent $\alpha$${\sim}$0.75) relation between the 3.3 $\mu$m PAH luminosity surface density and SFR traced by Br$\alpha$ for compact, cospatial (within 0.16$''$ or ${\sim}$7 pc) peaks in Pa$\alpha$, Br$\alpha$, and 3.3 $\mu$m (eYSC-I). The scatter in the relationship does not correlate well with variations in local interstellar medium (ISM) metallicity due to a radial metallicity gradient, but rather is likely due to stochastic sampling of the stellar initial mass function (IMF) and variations in the PAH heating and age of our sources. The deviation from a linear relation may be explained by PAH destruction in more intense ionizing environments, variations in age, and IMF stochasticity at intermediate to low luminosities. We test our results with various continuum subtraction techniques using combinations of NIRCam bands and find that they remain robust with only minor differences in the derived slope and intercept. An unexpected discrepancy is identified between the relations of hydrogen recombination lines (Pa$\alpha$ versus Br$\alpha$; H$\alpha$ versus Br$\alpha$).
... NGC 4631 shows a double-peaked central CO emission (Golla & Wielebinski 1994 ;Irwin et al. 2011 ), with a similar structure in the dust emission (Bendo et al. 2006 ). Irwin et al. ( 2011 ) suggested that the interaction between NGC 4631 and its companion galaxies agitates the disc and also initiates star formation. ...
We present the ${\rm H}{\rm \small {I}}$ distribution of galaxies from the Continuum Halos in Nearby Galaxies - an EVLA Survey (CHANG-ES). Though the observational mode was not optimized for detecting ${\rm H}{\rm \small {I}}$, we successfully produce ${\rm H}{\rm \small {I}}$ cubes for 19 galaxies. The moment-0 maps from this work are available on CHANG-ES data release website, i.e., https://www.queensu.ca/changes. Our sample is dominated by star-forming, ${\rm H}{\rm \small {I}}$-rich galaxies at distances from 6.27 to 34.1 Mpc. ${\rm H}{\rm \small {I}}$ interferometric images on two of these galaxies (NGC 5792 and UGC 10288) are presented here for the first time, while 12 of our remaining sample galaxies now have better ${\rm H}{\rm \small {I}}$ spatial resolutions and/or sensitivities of intensity maps than those in existing publications. We characterize the average scale heights of the ${\rm H}{\rm \small {I}}$ distributions for a subset of most inclined galaxies (inclination > 80 deg), and compare them to the radio continuum intensity scale heights, which have been derived in a similar way. The two types of scale heights are well correlated, with similar dependence on disk radial extension and star formation rate surface density but different dependence on mass surface density. This result indicates that the vertical distribution of the two components may be governed by similar fundamental physics but with subtle differences.
... NGC 4631 shows a double-peaked central CO emission (Golla & Wielebinski 1994;Irwin et al. 2011), with a similar structure in the dust emission (Bendo et al. 2006). Irwin et al. (2011) suggested that the interaction between NGC 4631 and its companion galaxies agitates the disk and also initiates star formation. ...
We present the HI distribution of galaxies from the Continuum Halos in Nearby Galaxies - an EVLA Survey (CHANG-ES). Though the observational mode was not optimized for detecting HI, we successfully produce HI cubes for 19 galaxies. The moment-0 maps from this work are available on CHANG-ES data release website, i.e., https://www.queensu.ca/changes. Our sample is dominated by star-forming, HI-rich galaxies at distances from 6.27 to 34.1 Mpc. HI interferometric images on two of these galaxies (NGC 5792 and UGC 10288) are presented here for the first time, while 12 of our remaining sample galaxies now have better HI spatial resolutions and/or sensitivities of intensity maps than those in existing publications. We characterize the average scale heights of the HI distributions for a subset of most inclined galaxies (inclination > 80 deg), and compare them to the radio continuum intensity scale heights, which have been derived in a similar way. The two types of scale heights are well correlated, with similar dependence on disk radial extension and star formation rate surface density but different dependence on mass surface density. This result indicates that the vertical distribution of the two components may be governed by similar fundamental physics but with subtle differences.
... our sample of galaxies. This is may be due to the fact that NGC 4631 is an edge-on galaxy (Bendo et al. 2006) and the cold dust can efficiently absorb the stellar radiation and re-emit it at IR wavelengths. It is a mild starburst galaxy, and may be in a late stage of its interaction (Dumke et al. 2004), which may increase its total IR luminosity. ...
We select 13 nearby spiral galaxies from the Nearby Galaxies Legacy Survey (NGLS) project and perform spectral energy distribution fitting for each galaxy applying two-component modified blackbody models on a global scale aim to probe the potential submillimeter (submm) excess. We find that NGC 2976, NGC 3351, and NGC 4631 show excess emission at 850 μm when using βc = 2.0. The contributions of CO(3–2), free–free emission or synchrotron radiation cannot explain their 850 μm excess. Our results suggest that a submm excess at 850 μm may be more easily detected for galaxies with faint total infrared luminosity and low cold dust mass. The colder temperature of cold dust, the more radiation of dust there is at 850 μm. The submm excess are prone to be detected in spiral galaxies with low stellar mass. As the metallicity of galaxies become poor, the submm excess is more obvious.
... However, 48 per cent of the sample have estimates of β below 1.5, which may be an indication of a submm excess, (i.e. an excess emission at wavelengths ≥500 μm with respect to what models would predict from the 100-350 μm wavelength range). Such an excess has been observed in dwarf galaxies (Lisenfeld et al. 2002;Galliano et al. 2003), in late-type galaxies (Dumke, Krause & Wielebinski 2004;Bendo et al. 2006;Galametz et al. 2009;Rela no et al. 2018), as well as in the Magellanic Clouds Israel et al. 2010;Planck Collaboration XVII 2011;Gordon et al. 2014). Several explanations have been proposed including a very cold dust component (<10 K), spinning dust (Anderson & Watson 1993;Draine & Lazarian 1998a,b), the Two-Level-System of amorphous dust (Meny et al. 2007), or a broken dust emissivity law model (where the emissivity changes at a 'break wavelength ' Li & Draine 2001). ...
We present the SCUBA-2 850 µm component of JINGLE, the new JCMT large survey for dust and gas in nearby galaxies, which with 193 galaxies is the largest targeted survey of nearby galaxies at 850 µm. We provide details of our SCUBA-2 data reduction pipeline, optimised for slightly extended sources, and including a calibration model adjusted to match conventions used in other far-infrared data. We measure total integrated fluxes for the entire JINGLE sample in 10 infrared/submillimetre bands, including all WISE, Herschel-PACS, Herschel-SPIRE and SCUBA-2 850 µm maps, statistically accounting for the contamination by CO(J=3-2) in the 850 µm band. Of our initial sample of 193 galaxies, 191 are detected at 250 µm with a ≥ 5σ significance. In the SCUBA-2 850 µm band we detect 126 galaxies with ≥ 3σ significance. The distribution of the JINGLE galaxies in far-infrared/sub-millimetre colour-colour plots reveals that the sample is not well fit by single modified-blackbody models that assume a single dust-emissivity index (β). Instead, our new 850 µm data suggest either that a large fraction of our objects require β < 1.5, or that a model allowing for an excess of sub-mm emission (e.g., a broken dust emissivity law, or a very cold dust component 10 K) is required. We provide relations to convert far-infrared colours to dust temperature and β for JINGLE-like galaxies. For JINGLE the FIR colours correlate more strongly with star-formation rate surface-density rather than the stellar surface-density, suggesting heating of dust is greater due to younger rather than older stellar-populations, consistent with the low proportion of early-type galaxies in the sample.
... However, 48% of the sample have estimates of β below 1.5, which may be an indication of a submm excess, (i.e., an excess emission at wavelengths ≥ 500 µm with respect to what models would predict from the 100-350 µm wavelength range). Such an excess has been observed in dwarf galaxies (Lisenfeld et al. 2002;Galliano et al. 2003), in late-type galaxies (Dumke et al. 2004;Bendo et al. 2006;Galametz et al. 2009;Relaño et al. 2018), as well as in the Magellanic Clouds Bot et al. 2010;Planck Collaboration et al. 2011;Gordon et al. 2014). System of amorphous dust (Meny et al. 2007), or a broken dust emissivity law model (where the emissivity changes at a 'break wavelength ' Li & Draine 2001). ...
We present the SCUBA-2 850 ${\mu}m$ component of JINGLE, the new JCMT large survey for dust and gas in nearby galaxies, which with 193 galaxies is the largest targeted survey of nearby galaxies at 850 ${\mu}m$. We provide details of our SCUBA-2 data reduction pipeline, optimised for slightly extended sources, and including a calibration model adjusted to match conventions used in other far-infrared data. We measure total integrated fluxes for the entire JINGLE sample in 10 infrared/submillimetre bands, including all WISE, Herschel-PACS, Herschel-SPIRE and SCUBA-2 850 ${\mu}m$ maps, statistically accounting for the contamination by CO(J=3-2) in the 850 ${\mu}m$ band. Of our initial sample of 193 galaxies, 191 are detected at 250 ${\mu}m$ with a $\geq$ 5${\sigma}$ significance. In the SCUBA-2 850 ${\mu}m$ band we detect 126 galaxies with $\geq$ 3${\sigma}$ significance. The distribution of the JINGLE galaxies in far-infrared/sub-millimetre colour-colour plots reveals that the sample is not well fit by single modified-blackbody models that assume a single dust-emissivity index $(\beta)$. Instead, our new 850 ${\mu}m$ data suggest either that a large fraction of our objects require $\beta < 1.5$, or that a model allowing for an excess of sub-mm emission (e.g., a broken dust emissivity law, or a very cold dust component 10 K) is required. We provide relations to convert far-infrared colours to dust temperature and $\beta$ for JINGLE-like galaxies. For JINGLE the FIR colours correlate more strongly with star-formation rate surface-density rather than the stellar surface-density, suggesting heating of dust is greater due to younger rather than older stellar-populations, consistent with the low proportion of early-type galaxies in the sample.
... However, even though PAH emission has been adopted to calibrate ongoing star formation rate (SFR; e.g., Roussel et al. 2001;Flores et al. 2004;Wu et al. 2005;Zhu et al. 2008;Wen et al. 2014), a correlation between the emissions of PAHs and cold dust has also been discovered and studied (Haas et al. 2002;Boselli et al. 2004;Bendo et al. 2006Bendo et al. , 2008Lu & Helou 2008;Zhu & Wu 2015). Even for the MIR continuum, some studies (e.g., Calapa et al. 2014;Lu et al. 2014;Jones et al. 2015) have suggested that apart from being related to ongoing star formation, there is still residual MIR emission associated with photodissociation regions or much cooler regions excited by interstellar radiation fields (ISRFs) created by evolved stars. ...
The correlation between mid-infrared (MIR) and Hα luminosity (hereafter referred to as the MIR versus Hα correlation) has been investigated for years, and these approximate linear correlations have been applied to many studies to derive the ongoing star formation rate (SFR) for galaxies near and far. We present and analyze the correlations between rest-frame 12 and 22 μm and Hα luminosities for a large sample of star-forming galaxies with redshift ranging from 0.03 to 0.15 selected in the cross-matched SDSS DR7 and ALLWISE survey. For the galaxies located in a relatively narrow redshift bin (Δz ,F∼ ,F0.01), we find that the fitting slope of the MIR versus Hα correlation is always less than 1, and less than the slope derived for all the star-forming galaxies covering a broad redshift range (0.03 < z < 0.15). Additionally, the fitting intercept increases with redshift. We check the influence on the L[MIR] versus L[Hα] correlation from K- and aperture correction, the variation of either star formation history or morphology, and find their influence is not large enough to account for the inconsistency of the MIR versus Hα correlation individually. We also find that there is possible evolution for the SFR versus M∗ (stellar mass) correlation within the redshift coverage from 0.03 to 0.15. Finally, we warn that an unwanted error might be brought in if the MIR versus Hα correlation derived from the sample covering a large redshift interval has been applied, and indicate an explicit study is needed to establish an accurate, redshift-independent MIR versus Hα correlation.
... The widespread presence of circumgalactic gas components in NGC 4631 is also indicated by a variety of other multiwavelength observations, from X-rays over Hα to other radio wavebands (Fabbiano & Trinchieri 1987;Wang et al. 1995;Neininger & Dumke 1999;Martin & Kern 2001;Bendo et al. 2006). The detection of (hot) ionized gas far away from the galactic plane was seen as evidence that NGC 4631 drives a galactic superwind (e.g., Melo & Muñoz-Tuñón 2002;Yamasaki et al. 2009). ...
We present a detailed analysis of the absorption properties of one of the tidal gas streams around the "Whale" galaxy NGC 4631 in the direction of the quasar 2MASS J12421031+3214268. Our study is based on ultraviolet spectral data obtained with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST) and 21cm-data from the HALOGAS project and the Green Bank Telescope (GBT). We detect strong H i Lyα absorption in the velocity range +550 to +800 km s⁻¹ related to gas from a NGC 4631 tidal stream known as Spur 2. We measure a column density of log (N(H i/cm⁻²)) = 18.68 0.15, indicating that the quasar sightline traces the outer boundary of Spur 2 as seen in the 21 cm data. Metal absorption in Spur 2 is detected in the lines of O i, C ii, Si ii, and Si iii in a complex absorption pattern that reflects the multiphase nature of the gas. We find that the average neutral gas fraction in Spur 2 toward 2MASS J12421031+3214268 is only 14%. This implies that ionized gas dominates the total mass of Spur 2, which then may comprise more than 10⁹ M. No significant depletion of Si is observed, showing that Spur 2 does not contain significant amounts of dust. From the measured O i/H i column density ratio, we determine an α abundance in Spur 2 of solar ([α/H] = -0.900.16), which is substantially lower than what is observed in the NGC 4631 disk. The low metallicity and low dust content suggest that Spur 2 represents metal-deficient gas stripped off a gas-rich satellite galaxy during a recent encounter with NGC 4631. © 2018. The American Astronomical Society. All rights reserved.
... The presence of extraplanar components in NGC 4631 is reported by a variety of observations, from Xrays to radio wavebands (Fabbiano & Trinchieri 1987;Hummel et al. 1991;Rand 1994;Wang et al. 1995;Vogler & Pietsch 1996;Neininger & Dumke 1999;Martin & Kern 2001;Wang et al. 2001;Bendo et al. 2006) including the recent radio observation of molecular out flow (Irwin et al. 2011). In particular, Martin & Kern (2001) observed diffuse ionized gas far from the disk using H α imaging. ...
We present a deep CCD imaging in B and V bands which allows us to analyze the vertical structure of NGC 4631. We derive the scale heights of the thin and thick disks at a variety of positions along the major axis of the disk. The scale heights of the thin disk are nearly constant while those of the thick disk tend to increase with increasing galactocentric distance. The mean scale heights of the thin disk derived from B and V images are similar to each other (˜450 pc). Instead, those of the thick disk show a strong east-west asymmetry which is caused by the diffuse stellar emission that is most prominent in the north west regions above the disk plane. The ratio of scale heights (z_{thick}/z_{thin}) is about 2.5 in the east side of the disk. However, this ratio is greater than 4 for the thick disk above the disk plane in the west side of the galaxy.
We analyse the radio-to-submillimetre spectral energy distribution (SED) for the central pseudobulge of NGC 1365 using archival data from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Very Large Array (VLA). This analysis shows that free-free emission dominates the continuum emission at 50–120 GHz and produces about 75 per cent of the 103 GHz continuum emission. However, the fraction of 103 GHz continuum emission originating from free-free emission varies significantly among different subregions in the pseudobulge, particularly for an outflow from the AGN on the eastern pseudobulge where the synchrotron emission produces half of the 103 GHz continuum emission. Free-free emission also dominates at 103 GHz within the central 400 pc diameter region, but this emission is associated with the AGN rather than star formation. The star formation rate (SFR) within the pseudobulge derived from the ALMA free-free emission is 8.9 ± 1.1 M⊙ yr−1. This is comparable to the SFR from the mid-infrared emission but higher than the SFR from the extinction-corrected Hα line emission, mainly because the pseudobulge is heavily dust obscured. The 1.5 GHz emission yields a comparable SFR for the pseudobulge but may have lower SFRs within subregions of the pseudobulge because of the diffusion outside of these regions of the electrons producing the synchrotron radiation. We propose that applying a correction factor of 75 per cent to the 80–110 GHz continuum emission could provide valuable estimates of the free-free emission without performing any SED decomposition, which could derive extinction-free SFRs within 20 per cent accuracy.
Measurements of the star formation activity on cloud scales are fundamental to uncovering the physics of the molecular cloud, star formation, and stellar feedback cycle in galaxies. Infrared (IR) emission from small dust grains and polycyclic aromatic hydrocarbons (PAHs) is widely used to trace the obscured component of star formation. However, the relation between these emission features and dust attenuation is complicated by the combined effects of dust heating from old stellar populations and an uncertain dust geometry with respect to heating sources. We used images obtained with NIRCam and MIRI as part of the PHANGS–JWST survey to calibrate the IR emission at 21 μm, and the emission in the PAH-tracing bands at 3.3, 7.7, 10, and 11.3 μm as tracers of obscured star formation. We analysed ∼20 000 optically selected H II regions across 19 nearby star-forming galaxies, and benchmarked their IR emission against dust attenuation measured from the Balmer decrement. We modelled the extinction-corrected H α flux as the sum of the observed H α emission and a term proportional to the IR emission, with a IR as the proportionality coefficient. A constant a IR leads to an extinction-corrected H α estimate that agrees with those obtained with the Balmer decrement with a scatter of ∼0.1 dex for all bands considered. Among these bands, 21 μm emission is demonstrated to be the best tracer of dust attenuation. The PAH-tracing bands underestimate the correction for bright H II regions, since in these environments the ratio of PAH-tracing bands to 21 μm decreases, signalling destruction of the PAH molecules. For fainter H II regions, all bands suffer from an increasing contamination from the diffuse IR background. We present calibrations that take this effect into account by adding an explicit dependence on 2 μm emission or stellar mass surface density.
Cosmic dust grains are one of the fundamental ingredients of the interstellar medium (ISM). In spite of their small contribution to the total mass budget, dust grains play a significant role in the physical and chemical evolution of galaxies. Over the past decades, a plethora of multi-wavelength data, from UV to far-infrared, has increased substantially our knowledge on the dust properties of nearby galaxies. Nevertheless, one regime of the spectrum, the mm range, remains relatively unexplored. Thanks to the new, high-resolution data in the mm range observed with the NIKA2 instrument and our radiative transfer framework, we aim to firmly characterise the physical properties of the very cold dust (<15K), and to quantify the importance of different emission mechanisms in the mm. So far, we have developed a methodology to use dust radiative transfer modelling and applied it to a small group of face-on spiral galaxies. The combination of the new NIKA2 data with our radiative transfer techniques would provide the right conditions to generate an accurate model of the interplay between starlight and dust in a sizeable sample of spatiallyresolved nearby galaxies.
Context. The Magellanic Bridge is a tidal feature located between the Magellanic Clouds, containing young stars formed in situ. Its proximity allows high-resolution studies of molecular gas, dust, and star formation in a tidal low-metallicity environment.
Aims. Our goal is to characterize gas and dust emission in Magellanic Bridge A, the source with the highest 870 μ m excess of emission found in single-dish surveys.
Methods. Using the ALMA telescope including the Morita Array, we mapped a 3′ field of view centered on the Magellanic Bridge A molecular cloud, in 1.3 mm continuum emission and ¹² CO(2−1) line emission at subparsec resolution. This region was also mapped in continuum at 870 μ m and in ¹² CO(2−1) line emission at ~6 pc resolution with the APEX telescope. To study its dust properties, we also use archival Herschel and Spitzer data. We combine the ALMA and APEX ¹² CO(2−1) line cubes to study the molecular gas emission.
Results. Magellanic Bridge A breaks up into two distinct molecular clouds in dust and ¹² CO(2−1) emission, which we call North and South. Dust emission in the North source, according to our best parameters from fitting the far-infrared fluxes, is ≈3 K colder than in the South source in correspondence to its less developed star formation. Both dust sources present large submillimeter excesses in LABOCA data: according to our best fits the excess over the modified blackbody (MBB) fit to the Spitzer / Herschel continuum is E (870 μ m) ~ 7 and E (870 μ m) ~ 3 for the North and South sources, respectively. Nonetheless, we do not detect the corresponding 1.3 mm continuum with ALMA. Our limits are compatible with the extrapolation of the MBB fits, and therefore we cannot independently confirm the excess at this longer wavelength. The ¹² CO(2−1) emission is concentrated in two parsec-sized clouds with virial masses of around 400 and 700 M ⊙ . Their bulk volume densities are n (H 2 ) ~ 0.7−2.6 × 10 ³ cm ⁻³ , higher than typical bulk densities of Galactic molecular clouds. The ¹² CO luminosity to H 2 mass conversion factor α CO is 6.5 and 15.3 M ⊙ (K km s ⁻¹ pc ² ) ⁻¹ for the North and South clouds, calculated using their respective virial masses and ¹² CO(2−1) luminosities. Gas mass estimates from our MBB fits to dust emission yields masses M ~ 1.3 × 10 ³ M ⊙ and 2.9 × 10 ³ M ⊙ for North and South, respectively, a factor of ~4 higher than the virial masses we infer from ¹² CO.
Dwarf spheroidal galaxies are among the most numerous galaxy population in the Universe, but their main formation and evolution
channels are still not well understood. The three dwarf spheroidal satellites (NGC 147, NGC 185, and NGC 205) of the Andromeda
galaxy are characterised by very different interstellar medium (ISM) properties, which might suggest them being at different
galaxy evolutionary stages. While the dust content of NGC 205 has been studied in detail by De Looze et al. (2012), we present
new Herschel dust continuum observations of NGC 147 and NGC 185. The non-detection of NGC 147 in Herschel SPIRE maps puts a strong constraint on its dust mass (≤ 128$^{+124}_{-68}$ M⊙). For NGC 185, we derive a total dust mass Md = 5.1±1.0 × 103 M⊙, which is a factor of ∼ 2-3 higher than that derived from ISO and Spitzer observations and confirms the need for longer wavelength observations to trace more massive cold dust reservoirs. We, furthermore,
estimate the dust production by asymptotic giant branch (AGB) stars and supernovae (SNe). For NGC 147, the upper limit on
the dust mass is consistent with expectations of the material injected by the evolved stellar population. In NGC 185 and NGC 205,
the observed dust content is one order of magnitude higher compared to the estimated dust production by AGBs and SNe. Efficient
grain growth, and potentially longer dust survival times (3-6 Gyr) are required to account for their current dust content.
Our study confirms the importance of grain growth in the gas phase to account for the current dust reservoir in galaxies.
In this chapter, we present \(^{12}\text {CO}\) J = 3 \(\rightarrow \) 2 maps and analaysis of NGC 2976 and NGC 3351 obtained using the James Clerk Maxwell Telescope (JCMT), both early targets of the JCMT Nearby Galaxy Legacy Survey (NGLS).
We report the discovery of the closest collisional ring galaxy to the Milky Way. Such rare systems occur due to ‘bulls-eye’
encounters between two reasonably matched galaxies. The recessional velocity of about 840 km s−1 is low enough that it was detected in the Anglo-Australian Observatory/UK Schmidt Telescope Survey for Galactic H α emission.
The distance is only ∼10 Mpc and the main galaxy shows a full ring of star-forming knots, 6.1 kpc in diameter surrounding
a quiescent disc. The smaller assumed ‘bullet’ galaxy also shows vigorous star formation. The spectacular nature of the object
had been overlooked because of its location in the Galactic plane and proximity to a bright star and even though it is the
60th brightest galaxy in the HI Parkes All Sky Survey (HIPASS) H i survey. The overall system has a physical size of ∼15 kpc, a total mass of M* = 6.6 × 109 M⊙ (stars + H i), a metallicity of [O/H]∼ −0.4, and a star formation rate of 0.2–0.5 M⊙ yr−1, making it a Magellanic-type system. Collisional ring galaxies therefore extend to much lower galaxy masses than commonly
assumed. We derive a space density for such systems of 7 × 10− 5 Mpc− 3, an order of magnitude higher than previously estimated. This space density suggests Kathryn's Wheel is the nearest such
system. We present discovery images, CTIO 4-m telescope narrow-band follow-up images and spectroscopy for selected emission
components. Given its proximity and modest extinction along the line of sight, this spectacular system provides an ideal target
for future high spatial resolution studies of such systems and for direct detection of its stellar populations.
Context. Measuring star formation on a local scale is important to constrain star formation laws. It is not clear yet, however, whether and how the measure of star formation is affected by the spatial scale at which a galaxy is observed.
Aims. We wish to understand the impact of the resolution on the determination of the spatially resolved star formation rate (SFR) and other directly associated physical parameters such as the attenuation.
Methods. We carried out a multi-scale, pixel-by-pixel study of the nearby galaxy M 33. Assembling FUV, Hα, 8 μm, 24 μm, 70 μm, and 100 μm maps, we have systematically compared the emission in individual bands with various SFR estimators from a resolution of 33 pc to 2084 pc.
Results. There are strong, scale-dependent, discrepancies of up to a factor 3 between monochromatic SFR estimators and Hα+24 μm. The scaling factors between individual IR bands and the SFR show a strong dependence on the spatial scale and on the intensity of star formation. Finally, strong variations of the differential reddening between the nebular emission and the stellar continuum are seen, depending on the specific SFR (sSFR) and on the resolution. At the finest spatial scales, there is little differential reddening at high sSFR. The differential reddening increases with decreasing sSFR. At the coarsest spatial scales the differential reddening is compatible with the canonical value found for starburst galaxies.
Conclusions. Our results confirm that monochromatic estimators of the SFR are unreliable at scales smaller than 1 kpc. Furthermore, the extension of local calibrations to high-redshift galaxies presents non-trivial challenges because the properties of these systems may be poorly known.
We examine the relation between polycyclic aromatic hydrocarbon (PAH)
emission at 8 microns and far-infrared emission from hot dust grains at 24
microns and from large dust grains at 160 and 250 microns in the nearby spiral
galaxies NGC 2403 and M83 using data from the Spitzer Space Telescope and
Herschel Space Observatory. We find that the PAH emission in NGC 2403 is better
correlated with emission at 250 microns from dust heated by the diffuse
interstellar radiation field (ISRF) and that the 8/250 micron surface
brightness ratio is well-correlated with the stellar surface brightness as
measured at 3.6 microns. This implies that the PAHs in NGC 2403 are intermixed
with cold large dust grains in the diffuse interstellar medium (ISM) and that
the PAHs are excited by the diffuse ISRF. In M83, the PAH emission appears more
strongly correlated with 160 micron emission originating from large dust grains
heated by star forming regions. However, the PAH emission in M83 is low where
the 24 micron emission peaks within star forming regions, and enhancements in
the 8/160 micron surface brightness ratios appear offset relative to the dust
and the star forming regions within the spiral arms. This suggests that the
PAHs observed in the 8 micron band are not excited locally within star forming
regions but either by light escaping non-axisymmetrically from star forming
regions or locally by young, non-photoionising stars that have migrated
downstream from the spiral density waves. The results from just these two
galaxies show that PAHs may be excited by different stellar populations in
different spiral galaxies.
On the basis of observations by Spitzer and Herschel, we present and analyse the correlations between various monochromatic infrared (IR) luminosities for star-forming galaxies,
selected from two northern Spitzer Wide-area InfraRed Extragalactic Survey (SWIRE) fields. The 24- and 70-μm luminosities (L[24] and L[70]), which are dominated by the continuum of very small grains (VSGs) and warm dust in thermal equilibrium, respectively,
correlate tightly with ongoing star formation. The contribution from cool dust excited by evolved stars also increases as
the wavelength increases in the far-infrared (FIR) wavelength range. The spectral features of ionized polycyclic aromatic
hydrocarbons (PAHs) around rest-frame 8 μm are excited by the moderated radiation field related to evolved stars as well,
rather than by the intensive radiation field related to young stars. Even though the carriers of PAHs could be treated as
types of VSG with a smaller scale, the radiation condition between PAHs and classic VSGs seems to be significantly different.
The formulae to calculate the total infrared luminosity LTIR using L[8(dust)] and L[24] are re-scaled and we find that the L[8(dust)] (L[24]) formula likely underestimates (overestimates) LTIR for galaxies with unusualcurrent star formation activity.
To assess the effects of the cluster environment on the different components
of the interstellar medium, we analyse the FIR-submm properties of a sample of
star-forming dwarf (SFD) galaxies detected by the Herschel Virgo Cluster Survey
(HeViCS). We determine dust masses and dust temperatures by fitting a modified
black body (MBB) function to the spectral energy distributions (SEDs). Stellar
and gas masses, star formation rates (SFRs), and metallicities are obtained
from the analysis of a set of ancillary data. Dust is detected in 49 out of 140
optically identified dwarfs covered by the HeViCS field; considering only
dwarfs brighter than $m_B$ = 18 mag, this gives a detection rate of 43%. After
evaluating different emissivity indices, we find that the FIR-submm SEDs are
best-fit by $\beta$=1.5, with a median dust temperature $T_d$ = 22.4 K.
Assuming $\beta$=1.5, 67% of the 23 galaxies detected in all five Herschel
bands show emission at 500 $\mu$m in excess of the MBB model. The excess is
inversely correlated with SFR and stellar masses. To study the variations in
the global properties of our sample due to environmental effects, we compare
the Virgo SFDs to other Herschel surveys, such as KINGFISH, the Dwarf Galaxy
Survey (DGS), and the HeViCS bright galaxy catalogue (BGC). We explore the
relations between stellar mass and HI fraction, specific SFR, dust fraction,
gas-to-dust ratio over a wide range of stellar masses. Highly HI-deficient
Virgo dwarf galaxies are mostly characterised by quenched star formation
activity and lower dust fractions giving hints for dust stripping in cluster
dwarfs. However, we find that the fraction of dust removed has to be less than
that of the HI component. Since the Virgo SFDs are likely to be crossing the
cluster for the first time, a longer timescale might be necessary to strip the
more centrally concentrated dust distribution.
We present a multiwavelength study of the OH Megamaser galaxy (OHMG)
IRAS16399-0937, based on new HST/ACS F814W and H$\alpha$+[NII] images and
archive data from HST, 2MASS, Spitzer, Herschel and the VLA. This system has a
double nucleus, whose northern (IRAS16399N) and southern (IRAS16399S)
components have a projected separation of $\sim$ 6'' (3.4 kpc) and have
previously been identified based on optical spectra as a Low Ionization Nuclear
Emission Line Region (LINER) and starburst nucleus, respectively. The nuclei
are embedded in a tidally distorted common envelope, in which star formation is
mostly heavily obscured. The infrared spectrum is dominated by strong
polycyclic aromatic hydrocarbon (PAH), but deep silicate and molecular
absorption features are also present, and are strongest in the IRAS16399N
nucleus. The 0.435 - 500$\mu$m SED was fitted with a model including stellar,
ISM and AGN torus components using our new MCMC code, clumpyDREAM. The results
indicate that the IRAS16399N contains an AGN (L$_{bol} \sim 10^{44}$ ergs/s)
deeply embedded in a quasi-spherical distribution of optically-thick clumps
with a covering fraction $\approx1$. We suggest that these clumps are the
source of the OHM emission in IRAS16399-0937. The high torus covering fraction
precludes AGN-photoionization as the origin of the LINER spectrum, however, the
spectrum is consistent with shocks (v $\sim100-200$ km s$^{-1}$). We infer that
the $\sim10^8$ M$_{\odot}$ black-hole in IRAS16399N is accreting at a small
fraction ($\sim1$%) of its Eddington rate. The low accretion-rate and modest
nuclear SFRs suggest that while the gas-rich major merger forming the
IRAS16399-0937 system has triggered widespread star formation, the massive gas
inflows expected from merger simulations have not yet fully developed.
The James Clerk Maxwell Telescope Nearby Galaxies Legacy Survey (NGLS) comprises an H i-selected sample of 155 galaxies spanning all morphological types with distances less than 25 Mpc. We describe the scientific
goals of the survey, the sample selection and the observing strategy. We also present an atlas and analysis of the CO J=3 - 2 maps for the 47 galaxies in the NGLS which are also part of the Spitzer Infrared Nearby Galaxies Survey. We find a wide range of molecular gas mass fractions in the galaxies in this sample and
explore the correlation of the far-infrared luminosity, which traces star formation, with the CO luminosity, which traces
the molecular gas mass. By comparing the NGLS data with merging galaxies at low and high redshift, which have also been observed
in the CO J=3 - 2 line, we show that the correlation of far-infrared and CO luminosity shows a significant trend with luminosity. This
trend is consistent with a molecular gas depletion time which is more than an order of magnitude faster in the merger galaxies
than in nearby normal galaxies. We also find a strong correlation of the LFIR/LCO(3-2) ratio with the atomic-to-molecular gas mass ratio. This correlation suggests that some of the far-infrared emission originates
from dust associated with atomic gas and that its contribution is particularly important in galaxies where most of the gas
is in the atomic phase.
We examine the UV reprocessing efficiencies of warm dust and polycyclic
aromatic hydrocarbons (PAHs) through an analysis of the mid- and far-infrared
surface luminosity densities of 85 nearby H$\alpha$-selected star-forming
galaxies detected by the volume-limited KPNO International Spectroscopic Survey
(KISS). Because H$\alpha$ selection is not biased toward continuum-bright
objects, the KISS sample spans a wide range in stellar masses
($10^8$-$10^{12}\rm{M}_\odot$), as well as H$\alpha$ luminosity
($10^{39}$-$10^{43}\rm{ergs/s}$), mid-infrared 8.0$\mu$m luminosity
($10^{41}$-$10^{44}\rm{ergs/s}$), and [Bw-R] color (-.1-2.2). We find that
mid-infrared polycyclic aromatic hydrocarbon (PAH) emission in the Spitzer IRAC
8.0$\mu$m band correlates with star formation, and that the efficiency with
which galaxies reprocess UV energy into PAH emission depends on metallicity. We
also find that the relationship between far-infrared luminosity in the Spitzer
MIPS 24$\mu$m band pass and H$\alpha$-measured star-formation rate varies from
galaxy to galaxy within our sample; we do not observe a metallicity dependence
in this relationship. We use optical colors and established mass-to-light
relationships to determine stellar masses for the KISS galaxies; we compare
these masses to those of nearby galaxies as a confirmation that the
volume-limited nature of KISS avoids strong biases. We also examine the
relationship between IRAC 3.6$\mu$m luminosity and galaxy stellar mass, and
find a color-dependent correlation between the two.
We investigate the far-infrared (IR) dust emission for 20 local star forming
galaxies from the Key Insights on Nearby Galaxies: A Far-IR Survey with
Herschel (KINGFISH) sample. We model the far-IR/submillimeter spectral energy
distribution (SED) using images from Spitzer Space Telescope and Herschel Space
Observatory. We calculate the cold dust temperature (T(cold)) and emissivity
(beta) on a pixel by pixel basis (where each pixel ranges from 0.1-3 kpc^2)
using a two temperature modified blackbody fitting routine. Our fitting method
allows us to investigate the resolved nature of temperature and emissivity
variations by modeling from the galaxy centers to the outskirts (physical
scales of ~15-50 kpc, depending on the size of the galaxy). We fit each SED in
two ways: (1) fit T(cold) and beta simultaneously, (2) hold beta constant and
fit T(cold). We compare T(cold) and beta with star formation rates (calculated
from L(Halpha) and L(24)), the luminosity of the old stellar population (traced
through L(3.6), and the dust mass surface density (traced by 500 micron
luminosity, L(500)). We find a significant trend between SFR/L(500) and
T(cold), implying that the flux of hard UV photons relative to the amount of
dust is significantly contributing to the heating of the cold, or diffuse, dust
component. We also see a trend between L(3.6)/L(500) and beta, indicating that
the old stellar population contributes to the heating at far-IR/submillimeter
wavelengths. Finally, we find that when beta is held constant, T(cold) exhibits
a strongly decreasing radial trend, illustrating that the shape of the far-IR
SED is changing radially through a galaxy, thus confirming on a sample almost
double in size the trends observed in Galametz et al. (2012).
There is direct and indirect evidence that cold dust ( T< 100 K) may
be present in the outflow of some starburst galaxies. However, the
geometry, energy and mass of this dusty superwind is virtually unknown
in the far-infrared wavelengths (70--500 μm). In particular there are
crucial questions regarding these super-winds: what are the physical
conditions under which the gas and dust finds itself? what is the extent
of the dust and gas and the importance for its survival? what is the
nature of the turbulence boundary arising from the wind interaction? We
present a detail analysis and comparison of very deep far-infrared
observations of two nearby Starburst galaxies, NGC 3079 and NGC 4631. We
compare the dust distribution between our Herschel images, Spitzer IRAC
4.5 and 8 μm and, MIPS 24 μm. We examine the dust physical
properties in the wind and halo of these galaxies.
We present 12CO J = 3→2 maps of NGC 2976 and NGC 3351 obtained with the James Clerk Maxwell Telescope (JCMT), both early targets of the JCMT
Nearby Galaxy Legacy Survey (NGLS). We combine the present observations with 12CO J = 1→0 data and find that the computed 12CO J = 3→2 to 12CO J = 1→0 line ratio (R31) agrees with values measured in other NGLS field galaxies. We compute the MH2 value and find that it is robust against the value of R31 used. Using H i data from The H i Nearby Galaxy Survey, we find a tight correlation between the surface density of H2 and star formation rate density for NGC 3351 when 12CO J = 3→2 data are used. Finally, we compare the 12CO J = 3→2 intensity with the polycyclic aromatic hydrocarbon (PAH) 8 μm surface brightness and find a good correlation in the
high surface brightness regions. We extend this study to include all 25 Spitzer Infrared Nearby Galaxies Survey galaxies within the NGLS sample and find a tight correlation at large spatial scales. We
suggest that both PAH 8 μm and 12CO J = 3→2 are likely to originate in regions of active star formation.
Because the 8 {\mu}m polycyclic aromatic hydrocarbon (PAH) emission has been
found to correlate with other well-known star formation tracers, it has widely
been used as a star formation rate (SFR) tracer. There are, however, studies
that challenge the accuracy and reliability of the 8 {\mu}m emission as a SFR
tracer. Our study, part of the Herschel M33 Extended Survey (HERM33ES) open
time key program, aims at addressing this issue by analyzing the infrared
emission from the nearby spiral galaxy M33 at the high spatial scale of 75 pc.
Combining data from the Herschel Space Observatory and the Spitzer Space
Telescope we find that the 8 {\mu}m emission is better correlated with the 250
{\mu}m emission, which traces cold interstellar gas, than with the 24 {\mu}m
emission. The L(8)/L(24) ratio is highly depressed in 24 {\mu}m luminous
regions, which correlate with known HII regions. We also compare our results
with the dust emission models by Draine & Li (2007). We confirm that the
depression of 8 {\mu}m PAH emission near star-forming regions is higher than
what is predicted by models; this is possibly an effect of increased stellar
radiation from young stars destroying the dust grains responsible for the 8
{\mu}m emission as already suggested by other authors. We find that the
majority of the 8 {\mu}m emission is fully consistent with heating by the
diffuse interstellar medium, similar to what recently determined for the dust
emission in M31 by Draine at al. (2013). We also find that the fraction of 8
{\mu}m emission associated with the diffuse interstellar radiation field ranges
between 60% and 80% and is 40% larger than the diffuse fraction at 24 {\mu}m.
Although it accounts only for a small fraction of the baryonic mass, dust has a profound impact on the physical processes at play in galaxies. Thus, to understand the evolution of galaxies, it is essential not only to characterize dust properties per se, but also in relation to global galaxy properties. To do so, we derive the dust properties of galaxies in a volume limited, K-band selected sample, the Herschel Reference Survey (HRS).
We gather infrared photometric data from 8 mu m to 500 mu m from Spitzer, WISE, IRAS, and Herschel for all of the HRS galaxies. Draine & Li (2007, ApJ, 663, 866) models are fit to the data from which the stellar contribution has been carefully removed. We find that our photometric coverage is sufficient to constrain all of the parameters of the Draine & Li models and that a strong constraint on the 20 60 mu m range is mandatory to estimate the relative contribution of the photo-dissociation regions to the infrared spectral energy distribution (SED). The SED models tend to systematically underestimate the observed 500 mu m flux densities, especially for low-mass systems. We provide the output parameters for all of the galaxies, i.e., the minimum intensity of the interstellar radiation field, the fraction of polycyclic aromatic hydrocarbon (PAH), the relative contribution of PDR and evolved stellar population to the dust heating, the dust mass, and the infrared luminosity. For a subsample of gas-rich galaxies, we analyze the relations between these parameters and the main integrated properties of galaxies, such as stellar mass, star formation rate, infrared luminosity, metallicity, H alpha and H-band surface brightness, and the far-ultraviolet attenuation. A good correlation between the fraction of PAH and the metallicity is found, implying a weakening of the PAH emission in galaxies with low metallicities and, thus, low stellar masses. The intensity of the diffuse interstellar radiation field and the H-band and H alpha surface brightnesses are correlated, suggesting that the diffuse dust component is heated by both the young stars in star-forming regions and the diffuse evolved population.
We use these results to provide a new set of infrared templates calibrated with Herschel observations on nearby galaxies and a mean SED template to provide the z = 0 reference for cosmological studies. For the same purpose, we place our sample on the SFR-M-* diagram. The templates are compared to the most popular infrared SED libraries, enlightening a large discrepancy between all of them in the 20 100 mu m range.
We model the infrared to submillimetre spectral energy distribution of 11 nearby galaxies of the Key Insights on Nearby Galaxies:
A Far-Infrared Survey with Herschel sample using Spitzer and Herschel data and compare model extrapolations at 870 μm (using different fitting techniques) with Large APEX BOlometer CAmera (LABOCA)
870 μm observations. We investigate how the differences between predictions and observations vary with model assumptions or
environment. At global scales, we find that modified blackbody models using realistic cold emissivity indices (βc = 2 or 1.5) are able to reproduce the 870 μm observed emission within the uncertainties for most of the sample. Low values
(βc < 1.3) would be required in NGC 0337, NGC 1512 and NGC 7793. At local scales, we observe a systematic 870 μm excess when
using βc = 2.0. The βc = 1.5 or the Draine & Li (2007) models can reconcile predictions with observations in part of the discs. Some of the remaining ‘excesses’ occur towards
the centres and can be partly or fully accounted for by non-dust contributions such as CO(3–2) or, to a lesser extent, free–free
or synchrotron emission. In three non-barred galaxies, the remaining excesses rather occur in the disc outskirts. This could
be a sign of a flattening of the submm slope (and decrease of the effective emissivity index) with radius in these objects.
Aims. The goal of this paper is to analyse the behaviour of the gas-to-dust mass ratio (G/D) of local Universe galaxies over a wide metallicity range. We especially focus on the low-metallicity part of the G/D vs metallicity relation and investigate several explanations for the observed relation and scatter.
Methods. We assembled a total of 126 galaxies, covering a 2 dex metallicity range and with 30% of the sample with 12 + log(O/H)
Gamma-ray bursts (GRBs) have been proposed as a tool to study star formation
in the Universe, so it is crucial to investigate whether their host galaxies
and immediate environments are in any way special compared with other
star-forming galaxies. Here we present spatially resolved maps of dust emission
of the host galaxy of the closest known GRB 980425 at z=0.0085 using our new
high-resolution observations from Herschel, APEX, ALMA and ATCA. We modeled the
spectral energy distributions of the host and of the star-forming region
displaying the Wolf-Rayet signatures in the spectrum (WR region), located 800
pc away from the GRB position. The host is characterised by low dust content
and high fraction of UV-visible star-formation, similar to other dwarf
galaxies. Such galaxies are abundant in the local universe, so it is not
surprising to find a GRB in one of them, assuming the correspondence between
the GRB rate and star-formation. The WR region contributes substantially to the
host emission at the far-infrared, millimeter and radio wavelengths and we
propose this to be a consequence of its high gas density. If dense environments
are also found close to the positions of other GRBs, then the ISM density
should also be considered as an important factor influencing whether a given
stellar population can produce a GRB, in a similar way as metallicity.
The 41st annual student scientific conference "Physics of Space" aims to
overcome the gap between modern scientific research and basic education
in astronomy. The main purpose of the conference is to introduce the
students to modern problems and directions of scientific research. The
working languages of the conference are Russian and English.
We study the mid-infrared color space of 30 galaxies from the Spitzer Infrared Nearby Galaxies Survey (SINGS) survey for which Sloan Digital Sky Survey data are also available. We construct two-color maps for each galaxy and compare them to results obtained from combining Maraston evolutionary synthesis models, galactic thermally pulsating asymptotic giant branch (TP-AGB) colors, and smooth star formation histories. For most of the SINGS sample, the spatially extended mid-IR emission seen by Spitzer in normal galaxies is consistent with our simple model in which circumstellar dust from TP-AGB stars dominates at 8 and 24 μm. There is a handful of exceptions that we identify as galaxies that have high star formation rates presumably with star formation histories that cannot be assumed to be smooth, or anemic galaxies, which were depleted of their H I at some point during their evolution and have very low ongoing star formation rates.
We report the results of a Spitzer infrared (IR) study of the Cosmic Eye, a strongly lensed, L*UV Lyman break galaxy (LBG) at z = 3.074. We obtained Spitzer mid-IR spectroscopy as well as MIPS 24 and 70 μm photometry. The Eye is detected with high significance at both 24 and 70 μm and, when including a flux limit at 3.5 mm, we estimate an IR luminosity of L IR = 8.3+4.7–4.4× 1011L ☉ assuming a magnification of 28± 3. This L IR is eight times lower than that predicted from the rest-frame ultraviolet properties assuming a Calzetti reddening law. This has also been observed in other young LBGs, and indicates that the dust reddening law may be steeper in these galaxies. The mid-IR spectrum shows strong polycyclic aromatic hydrocarbon (PAH) emission at 6.2 and 7.7 μm, with equivalent widths near the maximum values observed in star-forming galaxies at any redshift. The L PAH-to-L IR ratio lies close to the relation measured in local starbursts. Therefore, L PAH or L MIR may be used to estimate L IR, and thus star formation rate, of LBGs, whose fluxes at longer wavelengths are typically below current confusion limits. We also report the highest redshift detection of the 3.3 μm PAH emission feature. The PAH ratio, L 6.2/L 3.3 = 5.1 ± 2.7, and the PAH-to-LIR ratio, L 3.3/L IR = 8.5 ± 4.7 × 10–4, are both in agreement with measurements in local starbursts and ultraluminous infrared galaxies (ULIRGs), suggesting that this line may serve as a good proxy for L PAH or L IR at z > 3 with the James Webb Space Telescope.
Using mid-infrared (MIR) images of four photometric bands of the Infrared
Camera (IRC) onboard the AKARI satellite, S7 (7 um), S11 (11 um), L15 (15 um),
and L24 (24 um), we investigate the interstellar dust properties of the nearby
pair of galaxies M51 with respect to its spiral arm structure. The arm and
interarm regions being defined based on a spatially filtered stellar component
model image, we measure the arm-to-interarm contrast for each band. The
contrast is lowest in the S11 image, which is interpreted as that among the
four AKARI MIR bands the S11 image best correlates with the spatial
distribution of dust grains including colder components, while the L24 image
with the highest contrast traces warmer dust heated by star forming activities.
The surface brightness ratio between the bands, i.e. color, is measured over
the disk of the main galaxy, M51a, at 300 pc resolution. We find that the
distribution of S7/S11 is smooth and well traces the global spiral arm pattern
while L15/S11 and L24/S11 peak at individual HII regions. This result indicates
that the ionization state of PAHs is related to the spiral structure.
Comparison with observational data and dust models also supports the importance
of the variation in the PAH ionization state within the M51a disk. However, the
mechanism driving this variation is not yet clear from currently available data
sets. Another suggestion from the comparison with the models is that the PAH
fraction to the total dust mass is higher than previously estimated.
We present new photometric data from our Herschel Key Programme, the Dwarf
Galaxy Survey (DGS), dedicated to the observation of the gas and dust in 48
low-metallicity environments. They were observed with PACS and SPIRE onboard
Herschel at 70,100,160,250,350, and 500 microns. We focus on a systematic
comparison of the derived FIR properties (FIR luminosity, dust mass, dust
temperature and emissivity index) with more metal-rich galaxies and investigate
the detection of a potential submm excess. The data reduction method is adapted
for each galaxy to derive the most reliable photometry from the final maps.
PACS flux densities are compared with the MIPS 70 and 160 microns bands. We use
colour-colour diagrams and modified blackbody fitting procedures to determine
the dust properties of the DGS galaxies. We also include galaxies from the
Herschel KINGFISH sample, containing more metal-rich environments, totalling
109 galaxies. The location of the DGS galaxies on Herschel colour-colour
diagrams highlights the differences in global environments of low-metallicity
galaxies. The dust in DGS galaxies is generally warmer than in KINGFISH
galaxies (T_DGS~32 K, T_KINGFISH~23 K). The emissivity index, beta, is ~1.7 in
the DGS, but metallicity does not make a strong effect on beta. The
dust-to-stellar mass ratio is lower in low-metallicity galaxies:
M_dust/M_star~0.02% for the DGS vs 0.1% for KINGFISH. Per unit dust mass, dwarf
galaxies emit ~6 times more in the FIR than higher metallicity galaxies. Out of
the 22 DGS galaxies detected at 500 micron, 41% present an excess in the submm
not explained by our dust SED model. The excess mainly appears in lower
metallicity galaxies (12+log(O/H) < 8.3), and the strongest excesses are
detected in the most metal-poor galaxies. We stress the need for observations
longwards of the Herschel wavelengths to detect any submm excess appearing
beyond 500 micron.
The Sombrero galaxy (NGC 4594) is an Sa galaxy with a symmetric dust ring. We have used the Large APEX BOlometer CAmera (LABOCA) at 870 mum and the MAx-Planck Millimeter BOlometer (MAMBO-2) at 1.2 mm to detect the dust ring for the first time at submillimetre and millimetre wavelengths. We have constructed a model of the galaxy to separate the active galactic nucleus (AGN) and dust ring components. The ring radius at both 870 mum and 1.2 mm agrees well with the radius determined from optical absorption and atomic gas studies. The spectral energy distribution of the ring is well fitted by a single grey-body with dust emissivity index beta=2 and a dust temperature T_d=18.4 K. The dust mass of the ring is found to be 1.6±0.2× 107 M_&sun; which, for a Galactic gas-to-dust ratio, implies a gas mass that is consistent with measurements from the literature. This publication is based on data acquired with the IRAM 30 m telescope and the Atacama Pathfinder Experiment (APEX). APEX is a collaboration between the Max-Planck-Institut fur Radioastronomie, the European Southern Observatory, and the Onsala Space Observatory.
Aims. We study the physical properties and amount of molecular gas in the central regions of galaxies with active nuclei. Methods. Maps and measurements of the J=1-0, J=2-1, J=3-2, J=4-3 12CO, the J=1-0, J=2-1, and J=3-2 13CO lines in the central arcminute squared of NGC 1068, NGC 2146, NGC 3079, NGC 4826, and NGC 7469, as well as 492 GHz CI maps in three of these are used to model the molecular gas clouds in these galaxies. Results. Bright CO concentrations were detected and mapped in all five objects. In all cases, the observed lines could be fitted with two distinct gas components. The physical condition of the molecular gas is found to differ from galaxy to galaxy. Rather high kinetic temperatures of 125-150 K occur in NGC 2146 and NGC 3079. Very high densities of 0.3 1.0 × 105 cm−3 occur in NGC 2146, NGC 3079, and NGC 7469. The CO to H2 conversion factor X is typically an order of magnitude less than the 'standard' value in the Solar Neighborhood. The molecular gas is constrained within radii between 0.9 and 1.5 kpc from the nuclei. Within these radii, H2 masses are typically 1.2 2.5 × 108 M⊙. The exception is the (relatively nearby) merger NGC 4826 with R=0.3 kpc, and M = 3× 107 M⊙. The H2 mass is typically about one per cent of the dynamical mass in the same region.
We have examined dust photoelectric heating in the intergalactic medium (IGM). The heating rate in a typical radiation field of the IGM is represented by Γpe=1.2x10-34 erg s-1 cm-3 (D/10-4)(n
H/10-5 cm-3)43(J
L/10-21 erg s-1 cm-2 HZ-1 sr-1)2/3(T104 K)-1/6), where D is the dust-to-gas mass ratio, n
H is the hydrogen number density, J
L is the mean intensity at the hydrogen Lyman limit of the background radiation, and T is the gas temperature, if we assume the new X-ray photoelectric yield model by Weingartner et al. (2006) and the dust size distribution in the Milky Way by Mathis et al. (1977). This heating rate dominates the HI and HeII photoionization heating rates when the hydrogen number density is less than ∼10-6 cm-3 if D = 10-4 which is 1% of that in the Milky Way, although the heating rate is a factor of 2–4 smaller than that with the old yield model by Weingartner and Draine (2001). The grain size distribution is very important. If only large (≥0.1 μm) grains exist in the IGM, the heating rate is reduced by a factor of ≏5. Since dust heating is more efficient in a lower density medium relative to the photoionization heating, it may cause an inverted temperature-density relation in the low-density IGM, as suggested by Bolton et al. (2008). Finally, we have found that dust heating is not very important in the mean IGM before the cosmic reionization.
We review measurements of star formation rates (SFRs) at multiple wavelengths, comparing indicators commonly used in Milky Way studies with those used for external galaxies, and highlighting similarities and differences. Choices of the most appropriate SFR indicators for investigations in the two environments (Milky Way versus external galaxies) are driven by basic observational differences between the two, such as the dust column density (substantially larger towards the Galactic Center than towards a typical external, unresolved or partially resolved, galaxy) and the spatial resolution. Recent progress in the calibration of mid--infrared SFR indicators enabled by the Spitzer Space Telescope is reviewed with an outlook on the expected capabilities of future missions. SFR calibrations collected from the literature, from the X--ray to the radio, are presented after re--normalization to a common Kroupa stellar initial mass function.
We present the first complete CO J = 3–2 map of the nearby grand-design spiral galaxy M51 (NGC 5194), at a spatial resolution of ∼600 pc, obtained with the
HARP-B instrument on the James Clerk Maxwell Telescope. The map covers the entire optical galaxy disc and out to the companion
NGC 5195, with CO J = 3–2 emission detected over an area of ∼9 arcmin × 6 arcmin (∼21 × 14 kpc). We describe the CO J = 3–2 integrated intensity map and combine our results with maps of CO J = 2–1, CO J = 1–0 and other data from the literature to investigate the variation of the molecular gas, atomic gas and polycyclic aromatic
hydrocarbon (PAH) properties of M51 as a function of distance along the spiral structure on sub-kiloparsec scales. We find
that for the CO J = 3–2 and CO J = 2–1 transitions, there is a clear difference between the variation of arm and interarm emission with galactocentric radius,
with the interarm emission relatively constant with radius and the contrast between arm and interarm emission decreasing with
radius. For the CO J = 1–0 line and H i emission, the variation with radius shows a similar trend for the arm and interarm regions, and the arm–interarm contrast
appears relatively constant with radius. We investigate the variation of CO line ratios (J = 3–2/2–1, J = 2–1/1–0 and J = 3–2/1–0) as a function of distance along the spiral structure. Line ratios are consistent with the range of typical values
for other nearby galaxies in the literature. The highest CO J = 3–2/J = 2–1 line ratios are found in the central ∼1 kiloparsec and in the spiral arms and the lowest line ratios in the interarm
regions. We find no clear evidence of a trend with radius for the spiral arms, but for the interarm regions there appears
to be a trend for all CO line ratios to increase with radius. We find a strong relationship between the ratio of CO J = 3–2 intensity to stellar-continuum-subtracted 8 μm PAH surface brightness and the CO J = 3–2 intensity that appears to vary with radius.
We present new empirical calibrations to estimate resolved and integrated
total infrared luminosities from Spitzer and Herschel bands used as
monochromatic or combined tracers. We base our calibrations on resolved
elements of nearby galaxies (3 to 30 Mpc) observed with Herschel. We perform a
resolved SED modelling of these objects using the Draine and Li (2007) dust
models and investigate the influence of the addition of SPIRE measurements in
the estimation of LTIR. We find that using data up to 250 um leads to local
LTIR values consistent with those obtained with a complete coverage (up to 500
um) within 10 per cent for most of our resolved elements. We then study the
distribution of energy in the resolved SEDs of our galaxies. The bulk of energy
(30-50 per cent) is contained in the (70-160 um) band. The (24-70 um) fraction
decreases with increasing metallicity. The (160-1100 um) submillimeter band can
account for up to 25 per cent of the LTIR in metal-rich galaxies. We
investigate the correlation between TIR surface brightnesses/luminosities and
monochromatic Spitzer and Herschel surface brightnesses/luminosities. The three
PACS bands can be used as reliable monochromatic estimators of the LTIR, the
100 um band being the most reliable monochromatic tracer. There is also a
strong correlation between the SPIRE 250um and LTIR, although with more scatter
than for the PACS relations. We also study the ability of our monochromatic
relations to reproduce integrated LTIR of nearby galaxies as well as LTIR of
z=1-3 sources. Finally, we provide calibration coefficients that can be used to
derive TIR surface brightnesses/luminosities from a combination of Spitzer and
Herschel surface brightnesses/fluxes and analyse the associated uncertainties.
We present a study of the infrared/submm emission of the LMC star forming
complex N158-N159-N160. Combining observations from the Spitzer Space Telescope
(3.6-70um), the Herschel Space Observatory (100-500um) and LABOCA (870um)
allows us to work at the best angular resolution available now for an
extragalactic source. We observe a remarkably good correlation between SPIRE
and LABOCA emission and resolve the low surface brightnesses emission. We use
the Spitzer and Herschel data to perform a resolved Spectral Energy
Distribution (SED) modelling of the complex. Using MBB, we derive a global
emissivity index beta_c of 1.47. If beta cold is fixed to 1.5, we find an
average temperature of 27K. We also apply the Galliano et al. (2011) modelling
technique (and amorphous carbon to model carbon dust) to derive maps of the
star formation rate, the mean starlight intensity, the fraction of PAHs or the
dust mass surface density of the region. We observe that the PAH fraction
strongly decreases in the HII regions. This decrease coincides with peaks in
the mean radiation field intensity map. The dust surface densities follow the
FIR distribution, with a total dust mass of 2.1x10^4 Msolar (2.8 times less
than when using graphite grains) in the resolved elements we model. We find a
non-negligible amount of dust in the molecular cloud N159 South (showing no
massive SF). We also investigate the drivers of the Herschel/PACS and SPIRE
submm colours as well as the variations in the gas-to-dust mass ratio (G/D) and
the XCO conversion factor in the region N159. We finally model individual
regions to analyse variations in the SED shape across the complex and the 870um
emission in more details. No measurable submm excess emission at 870um seems to
be detected in these regions.
The goal of the present study is to establish the physical origin of dust
heating and emission based on radiation transfer models, which
self-consistently connect the emission components from diffuse dust and the
dust in massive star forming regions. NGC 4214 is a nearby dwarf galaxy with a
large set of ancillary data, ranging from the ultraviolet (UV) to radio,
including maps from SPITZER, HERSCHEL and detections from PLANCK. We mapped
this galaxy with MAMBO at 1.2 mm at the IRAM 30 m telescope. We extract
separate dust emission components for the HII regions (plus their associated
PDRs on pc scales) and for the diffuse dust (on kpc scales). We analyse the
full UV to FIR/submm SED of the galaxy using a radiation transfer model which
self-consistently treats the dust emission from diffuse and SF complexes
components, considering the illumination of diffuse dust both by the
distributed stellar populations, and by escaping light from the HII regions.
While maintaining consistency with the framework of this model we additionally
use a model that provides a detailed description of the dust emission from the
HII regions and their surrounding PDRs on pc scales. Due to the large amount of
available data and previous studies for NGC 4214 very few free parameters
remained in the model fitting process. We achieve a satisfactory fit for the
emission from HII+PDR regions on pc scales, with the exception of the emission
at 8\mi, which is underpredicted by the model. For the diffuse emission we
achieve a good fit if we assume that about 30-70% of the emission escaping the
HII+PDR regions is able to leave the galaxy without passing through a diffuse
ISM, which is not an unlikely scenario for a dwarf galaxy which has recently
undergone a nuclear starburst. We determine a dust-to-gas mass ratio of 350-390
which is close to the expected value based on the metallicity.
We present new 450 and 850 mum SCUBA data and 1.3 mm MAMBO data of the dwarf galaxies II Zw 40, He 2-10 and NGC 1140. Additional ISOCAM, IRAS as well as ground based data are used to construct the observed mid-infrared to millimeter spectral energy distribution of these galaxies. These spectral energy distributions are modeled in a self-consistent way, as was achieved with NGC 1569 (Galliano et al. 2003, A&A, 407, 159), synthesizing both the global stellar radiation field and the dust emission, with further constraints provided by the photoionisation of the gas. Our study shows that low-metallicity galaxies have very different dust properties compared to the Galaxy. Our main results are: (i) a paucity of PAHs which are likely destroyed by the hard penetrating radiation field; (ii) a very small ( 3{-}4 nm) average size of grains, consistent with the fragmentation and erosion of dust particles by the numerous shocks; (iii) a significant millimetre excess in the dust spectral energy distribution which can be explained by the presence of ubiquitous very cold dust (T = 5{-}9 K) accounting for 40 to 80% of the total dust mass, probably distributed in small clumps. We derive a range of gas-to-dust mass ratios between 300 and 2000, larger than the Galactic values and dust-to-metals ratios of 1/30 to 1/2. The modeled dust size distributions are used to synthesize an extinction curve for each galaxy. The UV slopes of the extinction curves resemble that observed in some regions in the Large Magellanic Cloud. The 2175 Å bumps of the modeled extinction curves are weaker than that of the Galaxy, except in the case of II Zw 40 where we are unable to accurately constrain the 2175 Å bump carrier.
We examine colors from 3.6 to 24 mum as a function of metallicity (O/H) for a sample of 34 galaxies. The galaxies range over 2 orders of magnitude in metallicity. They display an abrupt shift in the 8 mum-to-24 mum color for metallicities between one-third and one-fifth of the solar value. The mean 8-to-24 mum flux density ratio below and above 12+log(O/H)=8.2 is 0.08+/-0.04 and 0.70+/-0.53, respectively. We use mid-IR colors and spectroscopy to demonstrate that the shift is primarily due to a decrease in the 8 mum flux density, as opposed to an increase in the 24 mum flux density. This result is most simply interpreted as being due to a weakening at low metallicity of the mid-IR emission bands usually attributed to PAHs (polycyclic aromatic hydrocarbons) relative to the small-grain dust emission. However, existing empirical spectral energy distribution models cannot account for the observed short-wavelength (below 8 mum) colors of the low-metallicity galaxies merely by reducing the strength of the PAH features; some other emission source (e.g., hot dust) is required.
We have observed the continuum emission of the interacting galaxy NGC 4631 at lambdalambda 870 mum and 1.23 mm using the Heinrich-Hertz-Telescope on Mt. Graham and the IRAM 30-m telescope on Pico Veleta. We have obtained fully sampled maps which cover the optical emission out to a radius of about 7' at both wavelengths. For a detailed analysis, we carefully subtracted the line contributions and synchrotron and free-free emission from the data, which added up to 6% at 1.23 mm and 10% at 0.87 mm. We combined the flux densities with FIR data to obtain dust spectra and calculate dust temperatures, absorption cross sections, and masses. Assuming a ``standard'' dust model, which consists of two populations of big grains at moderate and warm temperatures, we obtained temperatures of 18 K and 50 K for the both components. However, such a model suffers from an excess of the radiation at lambda 1.23 mm, and the dust absorption cross section seems to be enhanced by a factor 3 compared to previous results and theoretiexpectations. At large galactocentric radii, where the galaxy shows disturbances as a result of gravitational interaction, this effect seems to be even stronger. Some possibilities to resolve these problems are discussed. The data could be explained by a very cold dust component at a temperature of 4-6 K, an increased abundance of very small grains, or a component of grains with unusual optical properties. We favour the latter possibility, since the first two lead to inconsistencies
The Infrared Array Camera (IRAC) is one of three focal plane instruments on the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broadband images at 3.6, 4.5, 5.8, and 8.0 μm. Two nearly adjacent 52 × 52 fields of view in the focal plane are viewed by the four channels in pairs (3.6 and 5.8 μm; 4.5 and 8 μm). All four detector arrays in the camera are 256 × 256 pixels in size, with the two shorter wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. IRAC is a powerful survey instrument because of its high sensitivity, large field of view, and four-color imaging. This paper summarizes the in-flight scientific, technical, and operational performance of IRAC.
We present multi-filter far infrared photometry of active and inactive
galaxies obtained with ISOPHOT. We find that the far infrared and
submillimeter spectrum of the active galaxies can be described by a
single modified black-body at a color temperature of 31.5 +/- 2.8 K. The
ratio of infrared luminosity to gas mass, L_IR/M_gas, where the latter
quantity has been obtained from 1.3 mm observations within the central
11'' is about 90 L_sun/ M_sun. In contrast, the spectral energy
distributions of inactive spirals require, apart from warm dust of 31.8
+/- 2.8 K, an additional very cold component of at most 12.9+/- 1.7 K.
Determining the gas mass from 1.3 mm dust continuum maps that cover the
optical extent of the inactive spirals we find L_IR/M_gas ~ 3 L_sun/
M_sun , a factor ~ 30 lower than for the active galaxies. Based on
observations with ISO, an ESA project with instruments funded by ESA
Member States (especially the PI countries: France, Germany, the
Netherlands and the United Kingdom) with the participation of ISAS and
NASA.
We present observations of the spiral galaxy M33 with Spitzer at 24, 70, and 160 μm. The excellent resolution and mapping capabilities of Spitzer combined with the proximity of M33 result in observations that enable a detailed study of the distribution of star formation (SF) and dust in the galaxy. We compare the morphology of M33 at far-infrared wavelengths with other standard SF indicators such as Hα and radio continuum using a Fourier filtering technique to separate the diffuse emission components from compact sources. We find that the infrared emission at 24 and 70 μm follows closely the structure of the ionized gas, indicating that it is heated largely by hot, ionizing stars. At 160 μm a diffuse cold dust component matches only approximately the structure of the old red stellar population or the distribution of blue light. It is, however, very similar to the structure of the diffuse nonthermal radio emission.
We present the largest galaxies as seen in the near-infrared (1–2 μm), imaged with the Two Micron All Sky Survey (2MASS), ranging in angular size from 1' to 15. We highlight the 100 largest in the sample. The galaxies span all Hubble morphological types, including elliptical galaxies, normal and barred spirals, and dwarf and peculiar classes. The 2MASS Large Galaxy Atlas provides the necessary sensitivity and angular resolution to examine in detail morphologies in the near-infrared, which may be radically different from those in the optical. Internal structures such as spirals, bulges, warps, rings, bars, and star formation regions are resolved by 2MASS. In addition to large mosaic images, the atlas includes astrometric, photometric, and shape global measurements for each galaxy. A comparison of fundamental measures (e.g., surface brightness, Hubble type) is carried out for the sample and compared with the Third Reference Catalogue. We further showcase NGC 253 and M51 (NGC 5194/5195) to demonstrate the quality and depth of the data. The atlas represents the first uniform, all-sky, dust-penetrated view of galaxies of every type, as seen in the near-infrared wavelength window that is most sensitive to the dominant mass component of galaxies. The images and catalogs are available through the NASA/IPAC Extragalactic Database and Infrared Science Archive and are part of the 2MASS Extended Source Catalog.
We examine far-infrared and submillimeter spectral energy distributions for galaxies in the Infrared Space Observatory Atlas of Bright Spiral Galaxies. For the 71 galaxies where we had complete 60–180 μm data, we fitted blackbodies with λ-1 emissivities and average temperatures of 31 K or λ-2 emissivities and average temperatures of 22 K. Except for high temperatures determined in some early-type galaxies, the temperatures show no dependence on any galaxy characteristic. For the 60–850 μm range in eight galaxies, we fitted blackbodies with λ-1, λ-2, and λ-β (with β variable) emissivities to the data. The best results were with the λ-β emissivities, where the temperatures were ~30 K and the emissivity coefficient β ranged from 0.9 to 1.9. These results produced gas-to-dust ratios that ranged from 150 to 580, which were consistent with the ratio for the Milky Way and which exhibited relatively little dispersion compared with fits with fixed emissivities.
We present images of the galaxy NGC 55 at 24, 70, and 160 μm obtained with the Multiband Imaging Photometer for Spitzer (MIPS) instrument on board the Spitzer Space Telescope. The new images display the far infrared emission in unprecedented detail and demonstrate that the infrared morphology differs dramatically from that at shorter wavelengths. The most luminous emission region in the galaxy is marginally resolved at 24 μm and has a projected separation of nearly 520 pc from the peak emission in the optical and near-infrared. This region is responsible for ~9% of the total emission at 24 μm and is likely a young star formation region. We show that this and other compact sources account for more than one-third of the total 24 μm emission. We compute a total infrared luminosity for NGC 55 of 1.2 × 109 L. The star formation rate implied by our measurements is 0.22 M yr-1. We demonstrate that the cold dust is more extended than the warm dust in NGC 55—the minor-axis scale heights are 0.32, 0.43, and 0.49 kpc at 24, 70, and 160 μm, respectively. The dust temperature map shows a range of temperatures that are well correlated with the 24 μm surface brightness, from 20 K in low surface brightness regions to 26 K in high surface brightness regions.
The Multiband Imaging Photometer for Spitzer (MIPS) provides long-wavelength capability for the mission in imaging bands at 24, 70, and 160 μm and measurements of spectral energy distributions between 52 and 100 μm at a spectral resolution of about 7%. By using true detector arrays in each band, it provides both critical sampling of the Spitzer point-spread function and relatively large imaging fields of view, allowing for substantial advances in sensitivity, angular resolution, and efficiency of areal coverage compared with previous space far-infrared capabilities. The 24 μm array has excellent photometric properties, and measurements with rms relative errors of about 1% can be obtained. The two longer-wavelength arrays use detectors with poor photometric stability, but a system of onboard stimulators used for relative calibration, combined with a unique data pipeline, produce good photometry with rms relative errors of less than 10%.
We present Spitzer observations of the nearby dwarf starburst galaxy NGC 1705 obtained as part of the Spitzer Infrared Nearby Galaxies Survey. The galaxy morphology is very different shortward and longward of ~5 μm: optical and short-wavelength IRAC imaging shows an underlying red stellar population, with the central super star cluster (SSC) dominating the luminosity; longer wavelength IRAC and MIPS imaging reveals warm dust emission arising from two off-nuclear regions that are offset by ~250 pc from the SSC and that dominate the far-IR flux of the system. These regions show little extinction at optical wavelengths. The galaxy has a relatively low global dust mass (~2 × 105 M☉, implying a global dust-to-gas mass ratio ~2-4 times lower than the Milky Way average, roughly consistent with the metallicity decrease). The off-nuclear dust emission appears to be powered by photons from the same stellar population responsible for the excitation of the observed Hα emission; these photons are unassociated with the SSC (although a contribution from embedded sources to the IR luminosity of the off-nuclear regions cannot be ruled out). Low-resolution IRS spectroscopy shows moderate-strength PAH emission in the 11.3 μm band in the more luminous eastern peak; no PAH emission is detected in the SSC or the western dust emission complex. There is significant diffuse emission in the IRAC 8 μm band after starlight has been removed by scaling shorter wavelength data; the fact that IRS spectroscopy shows spatially variable PAH emission strengths compared to the local continuum within this diffuse gas suggests caution in the interpretation of IRAC diffuse 8 μm emission as arising from PAH carriers alone. The nebular metallicity of NGC 1705 falls at the transition level of ~0.35 Z☉ found by Engelbracht and collaborators, below which PAH emission is difficult to detect; the fact that a system at this metallicity shows spatially variable PAH emission demonstrates the complexity of interpreting diffuse 8 μm emission in galaxies. NGC 1705 deviates significantly from the canonical far-infrared versus radio correlation, having significant far-infrared emission but no detected radio continuum.
The Spitzer Infrared Nearby Galaxies Survey (SINGS) is carrying out a comprehensive multiwavelength survey on a sample of 75 nearby galaxies. The 1-850 μm spectral energy distributions (SEDs) are presented using broadband imaging data from Spitzer, 2MASS, ISO, IRAS, and SCUBA. The infrared colors derived from the globally integrated Spitzer data are generally consistent with the previous generation of models that were developed using global data for normal star-forming galaxies, although significant deviations are observed. Spitzer's excellent sensitivity and resolution also allow a detailed investigation of the infrared SEDs for various locations within the three large, nearby galaxies NGC 3031 (M81), NGC 5194 (M51), and NGC 7331. A wide variety of spectral shapes is found within each galaxy, especially for NGC 3031, the closest of the three targets and thus the galaxy for which the smallest spatial scales can be explored. Strong correlations exist between the local star formation rate and the infrared colors fν(70 μm)/fν(160 μm) and fν(24 μm)/fν(160 μm), suggesting that the 24 and 70 μm emission are useful tracers of the local star formation activity level. Preliminary evidence indicates that variations in the 24 μm emission, and not variations in the emission from polycyclic aromatic hydrocarbons at 8 μm, drive the variations in the fν(8.0 μm)/fν(24 μm) colors within NGC 3031, NGC 5194, and NGC 7331. If the galaxy-to-galaxy variations in SEDs seen in our sample are representative of the range present at high redshift, then extrapolations of total infrared luminosities and star formation rates from the observed 24 μm flux will be uncertain at the factor of 5 level (total range). The corresponding uncertainties using the redshifted 8.0 μm flux (e.g., observed 24 μm flux for a z = 2 source) are factors of 10-20. Considerable caution should be used when interpreting such extrapolated infrared luminosities.
Far-ultraviolet to far-infrared images of the nearby galaxy NGC 5194 (M51a), from a combination of space-based (Spitzer, GALEX, and Hubble Space Telescope) and ground-based data, are used to investigate local and global star formation and the impact of dust extinction. The Spitzer data provide unprecedented spatial detail in the infrared, down to sizes ~500 pc at the distance of NGC 5194. The multiwavelength set is used to trace the relatively young stellar populations, the ionized gas, and the dust absorption and emission in H II-emitting knots, over 3 orders of magnitude in wavelength range. As is common in spiral galaxies, dust extinction is high in the center of the galaxy (AV ~ 3.5 mag), but its mean value decreases steadily as a function of galactocentric distance, as derived from both gas emission and stellar continuum properties. In the IR/UV-UV color plane, the NGC 5194 H II knots show the same trend observed for normal star-forming galaxies, having a much larger dispersion (~1 dex peak to peak) than starburst galaxies. We identify the dispersion as due to the UV emission predominantly tracing the evolved, nonionizing stellar population, up to ages ~50-100 Myr. While in starbursts the UV light traces the current star formation rate (SFR), in NGC 5194 it traces a combination of current and recent past SFRs. Possibly, mechanical feedback from supernovae is less effective at removing dust and gas from the star formation volume in normal star-forming galaxies than in starbursts because of the typically lower SFR densities in the former. The application of the starburst opacity curve for recovering the intrinsic UV emission (and deriving SFRs) in local and distant galaxies appears therefore appropriate only for SFR densities 1 M☉ yr-1 kpc-2. Unlike the UV emission, the monochromatic 24 μm luminosity is an accurate local SFR tracer for the H II knots in NGC 5194, with a peak-to-peak dispersion of less than a factor of 3 relative to hydrogen emission line tracers; this suggests that the 24 μm emission carriers are mainly heated by the young, ionizing stars. However, preliminary results show that the ratio of the 24 μm emission to the SFR varies by a factor of a few from galaxy to galaxy; this variation needs to be understood and carefully quantified before the 24 μm luminosity can be used as an SFR tracer for galaxy populations. While also correlated with star formation, the 8 μm emission is not directly proportional to the number of ionizing photons; it is overluminous, by up to a factor of ~2, relative to the galaxy's average in weakly ionized regions and is underluminous, by up to a factor of ~3, in strongly ionized regions. This confirms earlier suggestions that the carriers of the 8 μm emission are heated by more than one mechanism.
The Spitzer Space Telescope was used to study the mid- to far-infrared properties of NGC 300 and to compare dust emission to Hα to elucidate the heating of the interstellar medium (ISM) and the star formation cycle at scales smaller than 100 pc. The new data allow us to discern clear differences in the spatial distribution of 8 μm dust emission with respect to 24 μm dust and to H II regions traced by Hα light. The 8 μm emission highlights the rims of H II regions, and the 24 μm emission is more strongly peaked in star-forming regions than 8 μm. We confirm the existence and approximate amplitude of interstellar dust emission at 4.5 μm, detected statistically in Infrared Space Observatory (ISO) data, and conclude it arises in star-forming regions. When averaging over regions larger than ~1 kpc, the ratio of Hα to aromatic feature emission in NGC 300 is consistent with the values observed in disks of spiral galaxies. The mid- to far-infrared spectral energy distribution of dust emission is generally consistent with pre-Spitzer models.
The Spitzer Space Telescope, NASA's Great Observatory for infrared astronomy, was launched 2003 August 25 and is returning excellent scientific data from its Earth-trailing solar orbit. Spitzer combines the intrinsic sensitivity achievable with a cryogenic telescope in space with the great imaging and spectroscopic power of modern detector arrays to provide the user community with huge gains in capability for exploration of the cosmos in the infrared. The observatory systems are largely performing as expected, and the projected cryogenic lifetime is in excess of 5 years. This paper summarizes the on-orbit scientific, technical, and operational performance of Spitzer. Subsequent papers in this special issue describe the Spitzer instruments in detail and highlight many of the exciting scientific results obtained during the first 6 months of the Spitzer mission.
Well-resolved infrared observations of nearby galaxies are of fundamental importance to the study of the processes that affect galactic evolution. In this paper we report on the first imaging results from the Spitzer Infrared Nearby Galaxies Survey (SINGS) using observations of the Sb galaxy NGC 7331. We present images of NGC 7331 over a large range of wavelengths that allow us to compare the distributions of gas, stars, and dust in unprecedented detail. As an example of the types of information that the full SINGS will provide, we use three methods to determine that the interstellar medium mass in the ring of NGC 7331 is $5 ; 10 9 M . We also present the first images showing emission from small hot ($1000 K) dust grains, but we show that these dust grains contribute only a small fraction of the integrated 4.5 m emission from NGC 7331.
We present new 450 and 850$\,\mbox{$\mu$m}$ SCUBA data and 1.3$\,$mm MAMBO data of the dwarf galaxies II$\,$Zw$\,$40, He$\,$2-10 and NGC$\,$1140. Additional ISOCAM, IRAS as well as ground based data are used to construct the observed mid-infrared to millimeter spectral energy distribution of these galaxies. These spectral energy distributions are modeled in a self-consistent way, as was achieved with NGC$\,$1569 (Galliano et al. 2003, A&A, 407, 159), synthesizing both the global stellar radiation field and the dust emission, with further constraints provided by the photoionisation of the gas. Our study shows that low-metallicity galaxies have very different dust properties compared to the Galaxy. Our main results are: (i) a paucity of PAHs which are likely destroyed by the hard penetrating radiation field; (ii) a very small (~$3{-}4\,\rm nm$) average size of grains, consistent with the fragmentation and erosion of dust particles by the numerous shocks; (iii) a significant millimetre excess in the dust spectral energy distribution which can be explained by the presence of ubiquitous very cold dust ($T = 5{-}9$ K) accounting for 40 to 80% of the total dust mass, probably distributed in small clumps. We derive a range of gas-to-dust mass ratios between 300 and 2000, larger than the Galactic values and dust-to-metals ratios of $1/30$ to $1/2$. The modeled dust size distributions are used to synthesize an extinction curve for each galaxy. The UV slopes of the extinction curves resemble that observed in some regions in the Large Magellanic Cloud. The 2175 Å bumps of the modeled extinction curves are weaker than that of the Galaxy, except in the case of II$\,$Zw$\,$40 where we are unable to accurately constrain the 2175 Å bump carrier.
. We report photometry with ISO between 60 and 200¯m of six galaxies. Three of them are active and the extension of their spectra beyond the IRAS bands up to 200¯m confirms our previous conception that most of the dust there is warm and has a fairly uniform temperature of ¸30 K. In the other three galaxies which are inactive, we detect a very cold component (¸10 K and less) that could hitherto at best be surmised. The most dramatic consequence is a threefold increase in the mass of interstellar material over what one would calculate without the ISO data. Key words: galaxies: interstellar matter -- galaxies: dust 1. Introduction The gas content M gas is one of the fundamental parameters of a galaxy because it provides the reservoir for star formation. Probably the best method to estimate it is by observing around 1mm the optically thin emission of dust. The detected flux S and the total dust mass M d are directly proportional: M d = S =D 2 K B (T d ). Here K is the absorption coe...
We present spectroscopic observations of ionized gas in the disk-halo regions of five edge-on galaxies, covering a wavelength range from [OII] 3727A to [SII] 6716.4A. The inclusion of the [OII] emission provides additional constraints on the properties of the diffuse ionized gas (DIG), in particular, the origin of the observed spatial variations in the line intensity ratios. We have derived electron temperatures, ionization fractions and abundances along the slit. Our data include both slit positions parallel and perpendicular to the galactic disks. This allowed us to examine variations in the line intensity ratios with height above the midplane as well as distance from the galactic centers. The observed increase in the [OII]/Halpha line ratio towards the halo seems to require an increase in electron temperature caused by a non-ionizing heating mechanism. We conclude that gradients in the electron temperature can play a significant role in the observed variations in the optical emission line ratios from extraplanar DIG. Comment: accepted for publication in ApJ, 43 pages including 26 figures
The Halpha and mid-infrared mean disk surface brightnesses are compared in a sample of nearby spirals observed by ISOCAM. This shows that, in spiral disks, dust emission at 7 and 15 microns provides a reasonable star formation tracer. The fact that the 15 to 7 micron flux ratio is nearly constant in various global exciting conditions indicates a common origin, namely the aromatic infrared band carriers, and implies that at these wavelengths, dust emission from the disks of normal galaxies is dominated by photodissociation regions and not by HII regions themselves. We use this newly-found correlation between the mid-infrared and the Halpha line to investigate the nature of the link between the far-infrared (60 and 100 microns) and Halpha. Although the separation of the central regions from the disk is impossible to achieve in the far-infrared, we show that a circumnuclear contribution to the dust emission, having no equivalent counterpart in Halpha, is most likely responsible for the well-known non-linearity between far-infrared and Halpha fluxes in spiral galaxies. We derive a calibration of 7 and 15 micron fluxes in terms of star formation rates from a primary calibration of Halpha in the literature, and also outline the applicability limits of the proposed conversion, which should not be blindly extrapolated to objects whose nature is unknown.
We present new 450 and 850 microns SCUBA data of the dwarf galaxy NGC 1569. We construct the mid-infrared to millimeter SED of NGC 1569, using ISOCAM, ISOPHOT, IRAS, KAO, SCUBA and MAMBO data, and model the SED in order to explore the nature of the dust in low metallicity environments. The detailed modeling is performed in a self-consistent way, synthesizing the global ISRF of the galaxy using an evolutionary synthesis model with further constraints provided by the observed MIR ionic lines and a photoionisation model. Our results show that the dust properties are different in this low metallicity galaxy compared to other more metal rich galaxies. The results indicate a paucity of PAHs probably due to the destructive effects of the ISRF penetrating a clumpy environment and a size-segregation of grains where the emission is dominated by small grains of size ~3 nm, consistent with the idea of shocks having a dramatic effect on the dust properties in NGC 1569. A significant millimetre excess is present in the dust SED which can be explained by the presence of ubiquitous very cold dust (T = 5-7 K). This dust component accounts for 40 to 70 % of the total dust mass in the galaxy (1.6 - 3.4 10^5 Msol) and could be distributed in small clumps (size a few pc) throughout the galaxy. We find a gas-to-dust mass ratio of 740 - 1600, larger than that of the Galaxy and a dust-to-metals ratio of 1/4 to 1/7. We generate an extinction curve for NGC 1569, consistent with the modeled dust size distribution. This extinction curve has relatively steep FUV rise and smaller 2175 Angstroms bump, resembling the observed extinction curve of some regions in the Large Magellanic Cloud. Comment: 20 pages, 20 figures, accepted by A&A
The SIRTF Nearby Galaxy Survey is a comprehensive infrared imaging and
spectroscopic survey of 75 nearby galaxies. Its primary goal is to characterize
the infrared emission of galaxies and their principal infrared-emitting
components, across a broad range of galaxy properties and star formation
environments. SINGS will provide new insights into the physical processes
connecting star formation to the interstellar medium properties of galaxies,
and provide a vital foundation for understanding infrared observations of the
distant universe and ultraluminous and active galaxies. The galaxy sample and
observing strategy have been designed to maximize the scientific and archival
value of the data set for the SIRTF user community at large. The SIRTF images
and spectra will be supplemented by a comprehensive multi-wavelength library of
ancillary and complementary observations, including radio continuum, HI, CO,
submillimeter, BVRIJHK, H-alpha, Paschen-alpha, ultraviolet, and X-ray data.
This paper describes the main astrophysical issues to be addressed by SINGS,
the galaxy sample and the observing strategy, and the SIRTF and other ancillary
data products.
The metal deficient (Z = Z_sun/41) Blue Compact Dwarf Galaxy (BCD) SBS 0335-052 was observed with ISOCAM between 5 and 17 mic. With a L_12mic/L_B ratio of 2.15, the galaxy is unexpectedly bright in the mid-infrared for such a low-metallicity object. The mid-infrared spectrum shows no sign of the Unidentified Infrared Bands, which we interpret as an effect of the destruction of their carriers by the very high UV energy density in SBS 0335-052. The spectral energy distribution (SED) is dominated by a very strong continuum which makes the ionic lines of [SIV] and [NeIII] very weak. From 5 to 17 mic, the SED can be fitted with a grey-body spectrum, modified by an extinction law similar to that observed toward the Galactic Center, with an optical depth of A_V~19-21 mag. Such a large optical depth implies that a large fraction (as much as ~ 75%) of the current star-formation activity in SBS 0335-052 is hidden by dust with a mass between 3x10^3 M_sun and 5x10^5 M_sun. Silicate grains are present as silicate extinction bands at 9.7 and 18 mic can account for the unusual shape of the MIR spectrum of SBS 0335-052. It is remarkable that such a nearly primordial environment contains as much dust as galaxies which are 10 times more metal-rich. If the hidden star formation in SBS 0335-052 is typical of young galaxies at high redshifts, then the cosmic star formation rate derived from UV/optical fluxes would be underestimated. Comment: 13 pages, 4 figures, requires aaspp4.sty, accepted in ApJ
We describe the data reduction algorithms for the Multiband Imaging Photometer for Spitzer (MIPS) instrument. These algorithms were based on extensive preflight testing and modeling of the Si:As (24 micron) and Ge:Ga (70 and 160 micron) arrays in MIPS and have been refined based on initial flight data. The behaviors we describe are typical of state-of-the-art infrared focal planes operated in the low backgrounds of space. The Ge arrays are bulk photoconductors and therefore show a variety of artifacts that must be removed to calibrate the data. The Si array, while better behaved than the Ge arrays, does show a handful of artifacts that also must be removed to calibrate the data. The data reduction to remove these effects is divided into three parts. The first part converts the non-destructively read data ramps into slopes while removing artifacts with time constants of the order of the exposure time. The second part calibrates the slope measurements while removing artifacts with time constants longer than the exposure time. The third part uses the redundancy inherit in the MIPS observing modes to improve the artifact removal iteratively. For each of these steps, we illustrate the relevant laboratory experiments or theoretical arguments along with the mathematical approaches taken to calibrate the data. Finally, we describe how these preflight algorithms have performed on actual flight data. Comment: 21 pages, 16 figures, PASP accepted (May 2005 issue), version of paper with full resolution images is available at http://dirty.as.arizona.edu/~kgordon/papers/PS_files/mips_dra.pdf
We derive Galactic continuum spectra from 5-96/cm from COBE/FIRAS
observations. The spectra are dominated by warm dust emission, which may be fit
with a single temperature in the range 16-21 K (for nu^2 emissivity) along each
line of sight. Dust heated by the attenuated radiation field in molecular
clouds gives rise to intermediate temperature (10-14 K) emission in the inner
Galaxy only. A widespread, very cold component (4-7 K) with optical depth that
is spatially correlated with the warm component is also detected. The cold
component is unlikely to be due to very cold dust shielded from starlight,
because it is present at high latitude. We consider hypotheses that the cold
component is due to enhanced submillimeter emissivity of the dust that gives
rise to the warm component, or that it may be due to very small, large, or
fractal particles. Lack of substantial power above the emission from warm dust
places strong constraints on the amount of cold gas in the Galaxy. The
microwave sky brightness due to interstellar dust is dominated by the cold
component, and its angular variation could limit our ability to discern
primordial fluctuations in the cosmic microwave background radiation.
We combine samples of spiral galaxies and starburst systems observed with ISOCAM to investigate the reliability of mid-infrared (MIR) dust emission as a quantitative tracer of star formation (SF) activity. The total sample covers very diverse galactic environments and probes a much wider dynamic range in SF rate density than previous similar studies. We find that both the monochromatic 15 micron continuum and the 5-8.5 micron emission constitute excellent indicators of the SF rate as quantified by the Lyman continuum luminosity L_Lyc, within specified validity limits which are different for the two tracers. Normalized to projected surface area, the 15 micron luminosity S_15ct is directly proportional to S_Lyc over several orders of magnitude. Two regimes are distinguished from the relative offsets in the observed relation: the proportionality factor increases by a factor of ~ 5 between quiescent disks in spiral galaxies, and moderate to extreme star-forming environments in circumnuclear regions of spirals and in starburst systems. The transition occurs near S_Lyc ~ 10^2 L_sun pc^{-2} and is interpreted as due to very small dust grains starting to dominate the emission at 15 microns over aromatic species above this threshold. The 5-8.5 micron luminosity per unit area is also proportional to S_Lyc, with a single conversion factor from the most quiescent objects in the sample up to S_Lyc ~ 10^4 L_sun pc^{-2}, where the relation then flattens. The turnover is attributed to depletion of aromatic band carriers in the harsher conditions prevailing in extreme starbursts. The observed relations provide empirical calibrations useful for estimating SF rates from mid-IR observations in deeply embedded HII regions and obscured starbursts, and for predictions from evolutionary synthesis models. Comment: 17 pages, 3 tables, 5 figures -- accepted for publication in A&A
The Infrared Array Camera (IRAC) on the Spitzer Space Telescope is absolutely calibrated by comparing photometry on a set of A stars near the north ecliptic pole to predictions based on ground-based observations and a stellar atmosphere model. The brightness of point sources is calibrated to an accuracy of 3%, relative to models for A star stellar atmospheres, for observations performed and analyzed in the same manner as the calibration stars. This includes corrections for location of the star in the array and the location if the centroid within the peak pixel. Long-term stability of the IRAC photometry was measured by monitoring the brightness of A dwarfs and K giants (near the north ecliptic pole) observed several times per month; the photometry is stable to 1.5% (rms) over a year. Intermediate-time-scale stability of the IRAC photometry was measured by monitoring at least one secondary calibrator (near the ecliptic plane) every 12 hr while IRAC is in nominal operations; the intermediate-term photometry is stable with a 1% dispersion (rms). One of the secondary calibrators was found to have significantly time-variable (5%) mid-infrared emission, with period (7.4 days) matching the optical light curve; it is possibly a Cepheid variable.
We present maps of NGC 6946 at λ 61 and 205μm as well as scans
through the galaxy at these and at 8 intermediate wavelengths. The bulk
of the FIR luminosity arises from a diffuse disk component which extends
beyond the 25mag/arcsec^2^ B isophote to at least the radius
corresponding to the sensitivity limit. A FIR emission counterpart to
the outlying HI disk is seen at both 61 and 205μm, for which a dust
to HI mass ratio of 0.0046 is inferred from the 205μm brightness
assuming a dust temperature of 15K. The disk scale length at both
λ 61 and 205μm is similar to that seen in R-band.
This is the second in a series of papers presenting results from the SCUBA Local Universe Galaxy Survey. In our first paper
we provided 850-μm flux densities for 104 galaxies selected from the IRAS Bright Galaxy Sample and we found that the 60-, 100-μm (IRAS) and 850-μm (SCUBA) fluxes could be adequately fitted by emission from dust at a single temperature. In this paper we present
450-μm data for the galaxies. With the new data, the spectral energy distributions of the galaxies can no longer be fitted
with an isothermal dust model – two temperature components are now required. Using our 450-μm data and fluxes from the literature,
we find that the 450/850-μm flux ratio for the galaxies is remarkably constant, and this holds from objects in which the star
formation rate is similar to our own Galaxy, to ultraluminous infrared galaxies (ULIRGs) such as Arp 220. The only possible
explanation for this is if the dust emissivity index for all of the galaxies is ∼2 and the cold dust component has a similar
temperature in all galaxies . The 60-μm luminosities of the galaxies were found to depend on both the dust mass and the relative amount of energy in the
warm component, with a tendency for the temperature effects to dominate at the highest L60. The dust masses estimated using the new temperatures are higher by a factor of ∼2 than those determined previously using
a single temperature. This brings the gas-to-dust ratios of the IRAS galaxies into agreement with those of the Milky Way and other spiral galaxies which have been intensively studied in the
submm.
The Submillimetre Common-User Bolometer Array (SCUBA) is now operational
at the James Clerk Maxwell Telescope (JCMT). This paper describes the
SCUBA User Reduction Facility (SURF) data reduction software that has
been developed for use with SCUBA.
The recent star formation (SF) in the early-type spiral galaxy M81 is characterized using imaging observations from the far-ultraviolet to the far-infrared. We compare these data with models of the stellar, gas, and dust emission for subgalactic regions. Our results suggest the existence of a diffuse dust emission not directly linked to the recent star formation. We find a radial decrease of the dust temperature and dust mass density, and in the attenuation of the stellar light. The IR emission in M81 can be modeled with three components: (1) cold dust with a temperature = 18 ± 2 K, concentrated near the H II regions but also presenting a diffuse distribution; (2) warm dust with = 53 ± 7 K, directly linked with the H II regions; and (3) aromatic molecules, with diffuse morphology peaking around the H II regions. We derive several relationships to obtain total IR luminosities from IR monochromatic fluxes, and we compare five different star formation rate (SFR) estimators for H II regions in M81 and M51: the UV, Hα, and three estimators based on Spitzer data. We find that the Hα luminosity absorbed by dust correlates tightly with the 24 μm emission. The correlation with the total IR luminosity is not as good. Important variations from galaxy to galaxy are found when estimating the total SFR with the 24 μm or the total IR emission alone. The most reliable estimations of the total SFRs are obtained by combining the Hα emission (or the UV) and an IR luminosity (especially the 24 μm emission), which probe the unobscured and obscured SF, respectively. For the entire M81 galaxy, about 50% of the total SF is obscured by dust. The percentage of obscured SF ranges from 60% in the inner regions of the galaxy to 30% in the outer zones.
One of the major challenges to identification of the 3.3, 6.2, 7.7, 8.6, and 11.3 μm interstellar infrared (IR) emission bands with polycyclic aromatic hydrocarbon (PAH) molecules has been the recent detection of these bands in regions with little ultraviolet (UV) illumination, since small, neutral PAH molecules have little or no absorption at visible wavelengths and therefore require UV photons for excitation. We show here that our "astronomical" PAH model, incorporating the experimental result that the visual absorption edge shifts to longer wavelength upon ionization and/or as the PAH size increases, can closely reproduce the observed IR emission bands of vdB 133, a UV-poor reflection nebula. We also show that single-photon heating of "astronomical" PAHs in reflection nebulae near stars as cool as Teff = 3000 K can result in observable emission at 6.2, 7.7, 8.6, and 11.3 μm. Illustrative mid-IR emission spectra are calculated for reflection nebulae illuminated by cool stars with Teff = 3500, 4500, and 5000 K. These will allow comparison with future Space Infrared Telescope Facility observations of vdB 135 (Teff = 3600 K), vdB 47 (Teff = 4500 K), and vdB 101 (Teff = 5000 K). The dependence on the effective temperature of the exciting star of the observed 12 μm IRAS emission (relative to the total far-IR emission) is consistent with the PAH model for 3000 K ≤ Teff ≤ 30,000 K.
This is the first of a series of papers presenting results from the SCUBA Local Universe Galaxy Survey (SLUGS), the first statistical survey of the submillimetre properties of the local Universe. As the initial part of this survey, we have used the SCUBA camera on the James Clerk Maxwell Telescope to observe 104 galaxies from the IRAS Bright Galaxy Sample. We present here the 850-μm flux measurements.
The 60-, 100-, and 850-μm flux densities are well fitted by single-temperature dust spectral energy distributions, with the sample mean and standard deviation for the best-fitting temperature being Td=35.6±4.9 K and for the dust emissivity index β=1.3±0.2. The dust temperature was found to correlate with 60-μm luminosity. The low value of β may simply mean that these galaxies contain a significant amount of dust that is colder than these temperatures. We have estimated dust masses from the 850-μm fluxes and from the fitted temperature, although if a colder component at around 20 K is present (assuming a β of 2), then the estimated dust masses are a factor of 1.5–3 too low.
We have made the first direct measurements of the submillimetre luminosity function (LF) and of the dust mass function. Unlike the IRAS 60-μm LF, these are well fitted by Schechter functions. The slope of the 850-μm LF at low luminosities is steeper than −2, implying that the LF must flatten at luminosities lower than we probe here. We show that extrapolating the 60-μm LF to 850 μm using a single temperature and β does not reproduce the measured submillimetre LF. A population of ‘cold’ galaxies (Td<25 K) emitting strongly at submillimetre wavelengths would have been excluded from the 60-μm-selected sample. If such galaxies do exist, then this estimate of the 850-μm flux is biased (it is underestimated). Whether such a population does exist is unknown at present.
We correlate many of the global galaxy properties with the FIR/submillimetre properties. We find that there is a tendency for less luminous galaxies to contain hotter dust and to have a greater star formation efficiency (cf. Young). The average gas-to-dust ratio for the sample is 581±43 (using both the atomic and molecular hydrogen), which is significantly higher than the Galactic value of 160. We believe that this discrepancy is probably due to a ‘cold dust’ component at Td≤20 K in our galaxies. There is a surprisingly tight correlation between dust mass and the mass of molecular hydrogen, estimated from CO measurements, with an intrinsic scatter of ≃50 per cent.
Mid-infrared spectra and submillimetre maps are investigated for five galaxies covering a range of star forming activity. We find a good spatial coincidence between the 850 $\mu$m continuum emission and the strength of the PAH 7.7 $\mu$m line. The PAH 7.7 $\mu$m peak to 850 $\mu$m flux ratio lies in the range around 2 with a moderate dispersion across the galaxies. Both PAH and cold dust emission correlate also with the emission from very small grains at 14.3 $\mu$m, but regions with starbursts show an excess in the very small grain emission. This suggests that the PAH carriers are preferentially related to the regions dominated by cold dust and molecular clouds, where they are excited mainly by the interstellar radiation field. The lack of increased PAH/submm ratio in starburst knots suggests that starbursts play a minor role for powering the PAH emission in galaxies.
A catalog is presented of IRAS observations of 85 galaxies with blue-light isophotal diameters greater than 8 arcmin. The observations, data processing, and data measurement techniques are described, and total IRAS flux densities and integrated infrared emission properties of the sample are reported. Infrared brightness profiles of the detected galaxies and infrared surface brightness contour maps of the galaxies for which structural features were resolved are displayed. A classification scheme based on the degree of central concentration and spatial structure of the 60 micron emission of the best-resolved galaxies is proposed. The 60 micron and blue-light isophotal diameters of the largest galaxies are compared.
Infrared to millimetre spectral energy distributions have been obtained for 41 bright ultra-luminous infrared galaxies. The observations were carried out with ISOPHOT between 10 and 200 micron and supplemented for 16 sources with SCUBA at 450 and 850 micron and with SEST at 1.3 mm. In addition, seven sources were observed at 1.2 and 2.2 $\mu$m with the 2.2 m telescope on Calar Alto. These new SEDs represent the most complete set of infrared photometric templates obtained so far on ULIRGs in the local universe. Comment: 23 pages, 11 figures, accepted for publication in Astronomy & Astrophysics
We present a quantitative model for the infrared emission from dust in the diffuse interstellar medium. The model consists of a mixture of amorphous silicate grains and carbonaceous grains, each with a wide size distribution ranging from molecules containing tens of atoms to large grains > 1 um in diameter. We assume that the carbonaceous grains have polycyclic aromatic hydrocarbon (PAH)-like properties at very small sizes, and graphitic properties for radii a > 50 A. On the basis of recent laboratory studies and guided by astronomical observations, we propose "astronomical" absorption cross sections for use in modeling neutral and ionized PAHs from the far ultraviolet to the far infrared. We also propose modifications to the far-infrared emissivity of "astronomical silicate". We calculate energy distribution functions for small grains undergoing "temperature spikes" due to stochastic absorption of starlight photons, using realistic heat capacities and optical properties. Using a grain size distribution consistent with the observed interstellar extinction, we are able to reproduce the near-IR to submillimeter emission spectrum of the diffuse interstellar medium, including the PAH emission features at 3.3, 6.2, 7.7, 8.6, and 11.3um. The model is compared with the observed emission at high Galactic latitudes as well as in the Galactic plane, as measured by COBE and IRTS. We calculate infrared emission spectra for our dust model heated by a range of starlight intensities, and we provide tabulated dust opacities (extended tables available at http://www.astro.princeton.edu/~draine/dust/dustmix.html)
SCUBA, the Submillimetre Common-User Bolometer Array, built by the Royal Observatory Edinburgh for the James Clerk Maxwell
Telescope, is the most versatile and powerful of a new generation of submillimetre cameras. It combines a sensitive dual-waveband
imaging array with a three-band photometer, and is sky-background-limited by the emission from the Mauna Kea atmosphere at
all observing wavelengths from 350 μμto 2 mm. The increased sensitivity and array size mean that SCUBA maps close to 10 000
times faster than its single-pixel predecessor (UKT14). SCUBA is a facility instrument, open to the world community of users,
and is provided with a high level of user support. We give an overview of the instrument, describe the observing modes, user
interface and performance figures on the telescope, and present a sample of the exciting new results that have revolutionized
submillimetre astronomy.
We present and analyse high-quality SCUBA 850- and 450-um images of 14 local spiral galaxies, including the detection of dust well out into the extended disk in many cases. We use these data in conjunction with published far-IR flux densities from IRAS and ISO, and millimetre-wave measurements from ground-based facilities to deduce the global properties of the dust in these galaxies; in particular temperature and mass. We find that simple two-temperature greybody models of fixed dust emissivity index beta=2 and with typical temperatures of 25 < T(warm) < 40 K and 10 < T(cold) < 20 K provide good fits to the spectral energy distributions. The dust mass in the cold component correlates with the mass in atomic hydrogen and the mass in the warm component correlates with the mass in molecular hydrogen. These results thus fit the simple picture in which the cold dust is heated predominantly by the interstellar radiation field while the hot dust is heated predominantly by OB stars in more active regions, although we argue that there is some mixing. The mean gas-to-dust ratio is 120+/-60, very similar to that found within our own galaxy and roughly a factor of 10 lower than that derived from IRAS data alone. The gas-to-dust ratios in the warm, molecular component are on average higher than those in the cold, atomic component. We compare our modelling results with similar results for more luminous spiral galaxies selected at far-IR wavelengths and find that whilst the total dust mass distributions of the two samples are indistinguishable, they have significantly different dust temperature distributions in both the warm and cold components. We suggest that this difference...(abridged) Comment: MNRAS in press. 19 pages. Colour images degraded resolution - full resolution pdf file available at http://www.roe.ac.uk/~jas/ME1050rv.pdf
We are living in a dusty universe: dust is ubiquitously seen in a wide variety of astrophysical environments, ranging from circumstellar envelopes around cool red giants to supernova ejecta, from diffuse and dense interstellar clouds and star-forming regions to debris disks around main-sequence stars, from comets to interplanetary space to distant galaxies and quasars.
These grains, spanning a wide range of sizes from a few angstroms to a few micrometers, play a vital role in the evolution of galaxies as an absorber, scatterer, and emitter of electromagnetic radiation, as a driver for the mass loss of evolved stars, as an essential participant in the star and planet formation process, as an efficient catalyst for the formation of H2 and other simple molecules as well as complex organic molecules which may lead to the origins of life, as a photoelectric heating agent for the interstellar gas, and as an agent shaping the spectral appearance of dusty systems such as protostars, young stellar objects, evolved stars and galaxies.
In this review I focus on the dust grains in the space between stars (interstellar dust), with particular emphasis on the extinction (absorption plus scattering) and emission properties of cold submicron-sized “classical” grains which, in thermal equilibrium with the ambient interstellar radiation field, obtain a steady-state temperature of ~ 15–25 K, warm nano-sized (or smaller) “ultrasmall” grains which are, upon absorption of an energetic photon, transiently heated to temperatures as high as a few hundred to over 1000 K, and the possible existence of a population of very cold (<10 K) dust. Whether dust grains can really get down to “temperature” less than the 2.7 K cosmic microwave background radiation temperature will also be discussed. The robustness of the silicate-graphite-PAHs interstellar dust model is demonstrated by showing that the infrared emission predicted from this model closely matches that observed for the Milky Way, the Small Magellanic Cloud, and the ringed Sb galaxy NGC 7331.
New far-infrared and submillimeter data are used to solidify and to extend to long wavelengths the empirical calibration of the infrared spectral energy distribution (SED) of normal star-forming galaxies. As was found by Dale et al. (2001), a single parameter family, characterized by f_nu(60 microns)/f_nu(100 microns), is adequate to describe the range of normal galaxy spectral energy distributions observed by IRAS and ISO from 3 to 100 microns. However, predictions based on the first generation models at longer wavelengths (122 to 850 microns) are increasingly overluminous compared to the data for smaller f_nu(60 microns)/f_nu(100 microns), or alternatively, for weaker global interstellar radiation fields. After slightly modifying the far-infrared/submillimeter dust emissivity in those models as a function of the radiation field intensity to better match the long wavelength data, a suite of SEDs from 3 microns to 20 cm in wavelength is presented. Results from relevant applications are also discussed, including submillimeter-based photometric redshift indicators, the infrared energy budget and simple formulae for recovering the bolometric infrared luminosity, and dust mass estimates in galaxies. Regarding the latter, since galaxy infrared SEDs are not well-described by single blackbody curves, the usual methods of estimating dust masses can be grossly inadequate. The improved model presented herein is used to provide a more accurate relation between infrared luminosity and dust mass.
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