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-Average IRS spectrum of 13 starburst galaxies (IC 342, NGC 660, NGC 1097, NGC 1222, NGC 2146, NGC 3310, NGC 3556, NGC 4088, NGC 4194, NGC 4676, NGC 4818, NGC 7252, and NGC 7714). All spectra have been normalized to a flux density of one at 15µm before co-addition. The dotted line shows the ISO-SWS spectrum of M82 (Sturm et al. 2000) for comparison.
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We present 5-38 μm mid-infrared spectra at a spectral resolution of R ≈ 65-130 of a large sample of 22 starburst nuclei taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. The spectra show a vast range of starburst SEDs. The silicate absorption ranges from essentially no absorption to heavily obscured systems with an op...
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... list them here since they can be easily de- tected, even at R ≤ 100. However, the flux measure- ments of the fine-structure lines can be done much more accurately from the IRS high-resolution spectra, which is the subject of a complementary paper discussing the ionic properties of the ISM (Devost et al. 2006). ...
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... illustrated by the average starburst spectrum in Fig. 6, PAH combination-mode emission features are common in our starburst spectra. Inspection of the indi- vidual spectra shows that the 5.25µm feature is usually double-peaked due to blending with the 5.34µm [Fe II] line (e.g. NGC 1222 and NGC 3256). Likewise, the profile of the 5.70µm PAH feature is affected by the presence of the H 2 S(7) ...
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... gas may not be as abundant in starburst nuclei as in ULIRG nuclei. Absorption features of aliphatic hydrocarbons at 6.85 and 7.25µm are thought to be tracers of the diffuse ISM ( Chiar et al. 2000). These features are readily detected in the spectra of deeply obscured ULIRG nuclei such as IRAS 20100-4156 (Fig. 5). The average starburst spec- trum (Fig. 6), in contrast, does not show similarly pro- nounced structure and the 7µm range is dominated in- stead by the blend of 6.91µm H 2 S(5) and 6.99µm [Ar II]. However, individual starburst spectra show weak spec- tral structure in the 6.5 − 7.0µm range. At 6.65µm, a weak emission feature seems to be present, most no- tably in the spectra ...
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... spectra have been normalized to a flux density of unity at 15µm before averaging. Figure 6 shows the resulting "template spectrum" and Table 6 lists the "average" spectral properties derived from this composed spectrum. Figure 6 also shows the ISO-SWS spectrum of M82 ( Sturm et al. 2000), which is often being used as a star- burst template, for comparison. ...
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... 6 shows the resulting "template spectrum" and Table 6 lists the "average" spectral properties derived from this composed spectrum. Figure 6 also shows the ISO-SWS spectrum of M82 ( Sturm et al. 2000), which is often being used as a star- burst template, for comparison. Although the spectral slope longward of 15µm is very similar, the two spectra show several distinct differences: the ISO-SWS spectrum of M82 does not show the pronounced PAH complex around 17µm, it shows much stronger silicate absorp- tion, and the flux density shortward of 12µm is almost a factor of two lower that in our average starburst tem- plate. ...
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... deeply obscured lines of sight towards ULIRG nuclei ( Spoon et al. 2006), we attribute the 23µm feature to crystalline silicates (forsterite). Their detec- tion in NGC 4945 suggests that crystalline silicates are perhaps a more common component of the ISM and not just limited to ULIRG nuclei. Figure 11 compares the average starburst template from Fig. 6 to NGC 4945, the most extincted source within our sample. For comparison we also show the heavily embedded ULIRG IRAS 08572+3915 from Spoon et al. (2006). The usually rather shallow 18µm sil- icate band reduces the continuum by a large factor compared to the average starburst spectrum. While NGC 4945 has a similarly strong silicate ...
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... The deep silicate band originating in the Si-O stretching mode of amorphous silicates, centred at 9.7 m, can impact the band ratios (e.g. Brandl et al. 2006;Donnan et al. 2023). Indeed, depending on the depth of the silicate band, the impact on the 11.3 m feature can be more significant than on the 7.7 m. ...
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... A comprehensive understanding of the interstellar medium (ISM) is essential in unraveling the mechanisms that regulate star formation and how they differ across various regions of a galaxy (e.g., Abdurro'uf et al. 2022). One effective way to study the morphology of the ISM is through IR observations, as it can trace the emission from polycyclic aromatic hydrocarbons (PAHs) and dust continuum, which are closely related to star-forming regions (Wu et al. 2005;Brandl et al. 2006;Reddy et al. 2006;Tielens 2008). PAHs are organic molecules that are excited by ultraviolet (UV) radiation from young, massive stars, while dust continuum emission arises from thermal radiation emitted by dust particles heated by these same stars. ...
We study the morphological properties of mid-infrared selected galaxies at $1.0<z<1.7$ in the SMACS J0723.3-7327 cluster field, to investigate the mechanisms of galaxy mass assembly and structural formation at cosmic noon. We develop a new algorithm to decompose the dust and stellar components of individual galaxies by utilizing high-resolution images in the MIRI F770W and NIRCam F200W bands. Our analyses reveal that most galaxies in the stellar mass range ${\rm 10^{9.5}<M_*/M_\odot<10^{10.5}}$ have dust cores relatively compact compared to their stellar cores, whereas the most massive ($\rm{M_* \sim 10^{10.9}\,M_\odot}$) galaxy in our sample displays a comparably compact stellar core as to dust. The observed compactness of the dust component is potentially attributed to the presence of a (rapidly growing) massive bulge, in some cases associated with elevated star formation. Expanding the sample size through a joint analysis of multiple Cycle~1 deep-imaging programs can help to confirm the inferred picture. Our pilot study highlights that MIRI offers an efficient approach to studying the structural formation of galaxies from cosmic noon to the modern universe.
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... al ( 2012 ) Kirkpatrick et al. ( 2012 ) decompose MIR spectroscopy into AGN and SF components, computing MIR (rest frame 5-12 μm) AGN fractions for each source. Their spectra is fit with a three-component model including a local starburst composite to fit SF (Brandl et al. 2006 ), a pure power law for AGN, and applying an extinction curve (Draine 2003 ) to the AGN component. Applying an extinction curve can identify and model Silicate absorption features, allowing Featureless and Silicate AGN to be distinguished for strong AGNs. ...
UV-FIR spectral energy distribution (SED) modelling is an effective way to disentangle emission between star formation (SF) and active galactic nuclei (AGNs) in galaxies; however, this approach becomes uncertain for composite AGNs/SF galaxies that comprise 50–70 per cent of IR-samples. Cosmic X-ray background (XRB) models require a large fraction of obscured AGNs to reproduce the observed XRB peak, motivating reliable SED analyses in objects where the AGNs may be ‘buried’ in the galaxy and in the mid-IR to far-IR SED. In this paper, we study a 24 $\mu$m-selected (S24 > 100 $\mu$Jy) sample of 95 galaxies with $0~{{\ \rm per\ cent}} \lt f_{\mathrm{ MIR},\mathrm{ AGN}} \lt 100~{{\ \rm per\ cent}}$, 0.4 < z < 2.7, and 1011L⊙ < LIR < 1013L⊙. We test the performance of AGN models ranging in torus optical depth via SED fitting, comparing results with Spitzer mid-infrared spectroscopy and X-ray observations. The best-fitting torus optical depth can shed light on whether these galaxies host a luminous obscured AGN population. We find that permitting a broader AGN SED parameter space results in improved fit quality with higher optical depths, higher FIR AGN contributions, and higher LBol, impacting the bright end of the LBol luminosity function. Our results suggest there may be a population of dust-obscured composites that are bolometrically significant but have their AGNs mostly hidden in the mid-IR SED. If so, literature applications of SED fitting that often simplify AGN models or omit optically thick tori may largely underestimate AGN contribution from composite sources, as these sources are both numerous and have solutions sensitive to the assumed range of AGN models.
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UV-FIR SED modeling is an effective way to disentangle emission between star formation (SF) and active galactic nuclei (AGN) in galaxies; however, this approach becomes uncertain for composite AGN/SF galaxies that comprise 50-70% of IR-samples. Cosmic X-ray background (XRB) models require a large fraction of obscured AGN to reproduce the observed XRB peak, motivating reliable SED analyses in objects where the AGN may be ``buried" in the galaxy and in the mid-IR to far-IR SED. In this paper, we study a 24$\mu$m-selected ($S_{24}$ > 100$\mu$Jy) sample of 95 galaxies with $0 \% < f_{MIR,AGN} < 100 \%$, 0.4 < z < 2.7, and $10^{11}$L$_{\odot}$ < L$_{IR}$ < $10^{13}$L$_{\odot}$. We test the performance of AGN models ranging in torus optical depth via SED fitting, comparing results with Spitzer MIR spectroscopy and X-ray observations. Best-fit torus optical depth can shed light on whether these galaxies host a luminous obscured AGN population. We find that permitting a broader AGN SED parameter space results in improved fit quality with higher optical depths, higher FIR AGN contributions, and higher $L_{Bol}$, impacting the bright-end of the $L_{Bol}$ luminosity function. Our results suggest there may be a population of dust-obscured composites that are bolometrically significant but have their AGN mostly hidden in the mid-IR SED. If so, literature applications of SED fitting that often simplify AGN models or omit optically thick tori may largely underestimate AGN contribution from composite sources, as these sources are both numerous and have solutions sensitive to the assumed range of AGN models.
