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Many of the results in modern astrophysics rest on the notion that the initial mass function (IMF) is universal. Our observations of a sample of H I selected galaxies in the light of Hα and the far-ultraviolet (FUV) challenge this result. The extinction-corrected flux ratio F Hα/f FUV from these two tracers of star formation shows strong correlatio...
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Citations
... First, to be reconciled with the observed higher Eu/Fe ratio of metal-poor LMC stars compared to those for Galactic stars, we reduce the contribution of the two elements from CCSNe by using an IMF lacking very massive stars of >25 M e . The suppression of the formation of massive stars in low star formation galaxies is indicated by both observational and theoretical studies (e.g., Lee et al. 2009;Meurer et al. 2009;Pflamm-Altenburg et al. 2009). In the actual conditions, it is possible to consider that this assumed reduction in heavy elements for chemical enrichment could be, at least in part, due to their loss by galactic winds, which are likely to occur in dwarf galaxies (e.g., Matteucci & Tosi 1985;Bradamante et al. 1998). ...
The enrichment history of r -process elements has been imprinted on the stellar abundances that change in accordance with increasing metallicity in galaxies. Close examination of the [Eu/Fe] feature caused by stars in nearby galaxies, including the Large Magellanic Cloud (LMC), shows its perplexity. The decreasing trend of the [Eu/Fe] feature is followed by a nearly constant value; this trend is generally attributed to an onset of the delayed Fe release from Type Ia supernovae (SNe Ia), which is the same interpretation of the [ α /Fe] feature. However, this feature appears in the LMC at [Fe/H] of approximately −0.7, which is significantly higher than that for the [ α /Fe] case (≈−2). This result potentially indicates the presence of an overlooked property of the r -process site that remains unseen in the study of the Milky Way. Here, we propose that this [Eu/Fe]-knee feature is created by a fade-out of core-collapse SNe producing r -process elements; these elements along with neutron star mergers (NSMs) promote the r -process enrichment under the condition for this specific SNe such that their occurrence is limited to a low-metallicity environment. This metallicity threshold for the occurrence rate of r -process SNe at a subsolar is nearly identical to that for long gamma-ray bursts whose origin may be connected to fast-rotating massive stars. Moreover, we reason that the contribution of Eu from NSMs is crucial to maintain a high [Eu/Fe] at an early stage in dwarf galaxies by a balance with Fe from SNe Ia; both enrichments via NSMs and SNe Ia proceed with similar delay time distributions.
... With that in mind, one tentative explanation for an increase in the scatter seen in the lowest mass galaxies (e.g., Lee et al. 2007Lee et al. , 2009Meurer et al. 2009) relates to possible variations in the stellar IMF. We can speculatively draw on those results to consider whether differences in the IMF may also contribute to our result. ...
We demonstrate the importance of radio selection in probing heavily obscured galaxy populations. We combine Evolutionary Map of the Universe (EMU) Early Science data in the Galaxy and Mass Assembly (GAMA) G23 field with the GAMA data, providing optical photometry and spectral line measurements, together withWide-field Infrared Survey Explorer (WISE) infrared (IR) photometry, providing IR luminosities and colours. We investigate the degree of obscuration in star forming galaxies, based on the Balmer decrement (BD), and explore how this trend varies, over a redshift range of 0 < z < 0.345. We demonstrate that the radio detected population has on average higher levels of obscuration than the parent optical sample, arising through missing the lowest BD and lowest mass galaxies, which are also the lower star formation rate (SFR) and metallicity systems. We discuss possible explanations for this result, including speculation around whether it might arise from steeper stellar initial mass functions in low mass, low SFR galaxies.
... The H α-to-UV ratio has been used to quantitati vely e v aluate SFH burstiness for both local and high-z galaxies (e.g. Iglesias-P áramo et al. 2004 ;Meurer et al. 2009 ;Weisz et al. 2012 ;Guo et al. 2016 ;Emami et al. 2019 ;Faisst et al. 2019 ). Both the H α and far-UV luminosities of star-forming galaxies are known as tracers of star formation, but they trace star formation on different time-scales. ...
We use CANUCS JWST/NIRCam imaging of galaxies behind the gravitationally-lensing cluster MACS J0417.5-1154 to investigate star formation burstiness in low-mass (M⋆ ∼ 108 M⊙) galaxies at z ∼ 4.7 − 6.5. Our sample of 123 galaxies is selected using the Lyman break selection and photometric emission-line excess methods. Sixty percent of the 123 galaxies in this sample have Hα-to-UV flux ratios that deviate significantly from the range of Hα-to-UV ratio values consistent with smooth and steady star formation histories. This large fraction indicates that the majority of low-mass galaxies is experiencing bursty star formation histories at high redshift. We also searched for interacting galaxies in our sample and found that they are remarkably common ($\sim 40~{{\%}}$ of the sample). Compared to non-interacting galaxies, interacting galaxies are more likely to have very low Hα-to-UV ratios, suggesting that galaxy-galaxy interactions enhance star formation burstiness and enable faster quenching (with timescales of ≲ 100 Myr) that follows the rapid rise of star formation activity. Given the high frequency of galaxy-galaxy interactions and the rapid SFR fluctuations they appear to cause, we conclude that galaxy-galaxy interactions could be a leading cause of bursty star formation in low-mass, high-z galaxies. They could thus play a significant role in the evolution of the galaxy population at early cosmological times.
... Lowmass galaxies, which we typically define as having ( ) M M log 9.5 < , may be more susceptible to events of bursty SF, wherein SF is rapidly triggered and quenched on a timescale of tens of megayears, leading to processes such as expulsion of metal-enriched gas, etc. that can dramatically alter the chemical composition of a galaxy's ISM. Several studies have suggested that bursty SF is more prominent among low-mass or dwarf galaxies (e.g., Searle et al. 1973;Bell & Kennicutt 2001;Lee et al. 2009;Meurer et al. 2009;Weisz et al. 2012;Guo et al. 2016b), and theoretical models relating stellar mass and gas-phase metallicity with supernova-driven galactic winds also predict low-mass galaxies will exhibit more scatter in the MZR and FMR (Henry et al. 2013a(Henry et al. , 2013bLu et al. 2015;Guo et al. 2016a). But it is difficult to obtain comprehensive observations of the dwarf galaxy population at high redshift, since dwarf galaxies are faint at z > 0. Many higher redshift studies of the MZR are therefore either limited to the more massive population or probe only those dwarf galaxies with more extreme levels of SF and ionization. ...
We present gas-phase metallicity measurements for 583 emission line galaxies at 0.3 < z < 0.85, including 388 dwarf galaxies with log ( M ⋆ / M ⊙ ) < 9.5 , and explore the dependence of the metallicity on the stellar mass and star formation (SF) properties of the galaxies. Metallicities are determined through the measurement of emission lines in very deep (∼7 hr exposure) Keck/DEIMOS spectra taken primarily from the HALO7D survey. We measure metallicity with three strong-line calibrations (O3H β , R23, and O3O2) for the overall sample, as well as with the faint [Ne iii ] λ 3869 and [O iii ] λ 4363 emission lines for 112 and 17 galaxies where robust detections were possible. We construct mass–metallicity relations (MZR) for each calibration method, finding MZRs consistent with other strong-line results at comparable redshift, as well as with z ∼ 0 galaxies. We quantify the intrinsic scatter in the MZR as a function of mass, finding it increases with lower stellar mass. We also measure a weak but significant correlation between increased MZR scatter and higher specific star formation rate (SFR). We find a weak influence of SFR in the fundamental metallicity relation as well, with an SFR coefficient of α = 0.21. Finally, we use the flux ratios of the [O ii ] λ λ 3727,3729 doublet to calculate gas electron density in ∼1000 galaxies with log ( M ⋆ / M ⊙ ) < 10.5 as a function of redshift. We measure low electron densities ( n e ∼ 25 cm ⁻³ ) for z < 1 galaxies, again consistent with z ≈ 0 conditions, but measure higher densities ( n e ∼ 100 cm ⁻³ ) at z > 1. These results all suggest that there is little evolution in star-forming interstellar medium conditions from z ∼ 1 to z = 0, confirmed with a more complete sample of low-mass galaxies than has previously been available in this redshift range.
... Following equations (1)-(3) from Shi et al. (2018), we calculate the surface densities of SFR, atomic gas mass, and stellar mass, respectively. With the Kroupa initial mass function (Kroupa 2001), we use the FUV emission from GALEX which traces the young stellar population (Meurer et al. 2009) to calculate the SFR surface density: ...
The H i-rich ultra-diffuse galaxies (HUDGs) offer a unique case for studies of star formation laws (SFLs) as they host low star formation efficiency (SFE) and low-metallicity environments where gas is predominantly atomic. We collect a sample of six HUDGs in the field and investigate their location in the extended Schmidt law ($\Sigma _{\text{SFR }} \propto \left(\Sigma _{\text{star}}^{0.5} \Sigma _{\text{gas}}\right)^{1.09}$). They are consistent with this relationship well (with deviations of only 1.1 sigma). Furthermore, we find that HUDGs follow the tight correlation between the hydrostatic pressure in the galaxy mid-plane and the quantity on the x-axis ($\rm log(\Sigma _{star}^{0.5}\Sigma _{gas})$) of the extended Schmidt law. This result indicates that these HUDGs can be self-regulated systems that reach the dynamical and thermal equilibrium. In this framework, the stellar gravity compresses the disk vertically and counteracts the gas pressure in the galaxy mid-plane to regulate the star formation as suggested by some theoretical models.
... They conclude that an IMF that is deficient of massive stars in DGs and LSBs is consistent with their data, because the H α luminosity flattens at very low SFRs (Pflamm-Altenburg, Weidner & Kroupa 2007 ). This H α-to-FUV flux di vergence at lo w SFR is also documented by Meurer et al. ( 2009 ), as is the decreasing ratio of stellar ionizing to non-ionizing flux with smaller galaxy luminosity and mass, as well as lower SFR (Boselli et al. 2009 ;Meurer et al. 2009 ). Calzetti et al. ( 2010 ) find that a tendency towards a decreasing H α luminosity-to-cluster mass M cl exists, but not as strong as expected from (ii). ...
... They conclude that an IMF that is deficient of massive stars in DGs and LSBs is consistent with their data, because the H α luminosity flattens at very low SFRs (Pflamm-Altenburg, Weidner & Kroupa 2007 ). This H α-to-FUV flux di vergence at lo w SFR is also documented by Meurer et al. ( 2009 ), as is the decreasing ratio of stellar ionizing to non-ionizing flux with smaller galaxy luminosity and mass, as well as lower SFR (Boselli et al. 2009 ;Meurer et al. 2009 ). Calzetti et al. ( 2010 ) find that a tendency towards a decreasing H α luminosity-to-cluster mass M cl exists, but not as strong as expected from (ii). ...
When star clusters are formed at low star-formation rates (SFRs), their stellar initial mass function (IMF) can hardly be filled continuously with stars at each mass. This lack holds for massive stars and is verified observationally by the correlation between star-cluster mass and its most massive cluster star. Since galaxy evolution is strongly affected by massive stars, numerical models should account for this lack. Because a filled IMF is mostly applied even when only fractions of massive stars form, here we investigate, by means of 3D chemo-dynamical simulations of isolated dwarf galaxies, how deviations from a standard IMF in star clusters affect the evolution. We compare two different IMF recipes, a filled IMF and one truncated at a maximum mass at which a single complete star forms. Attention is given to energetic and chemical feedback by massive stars. Since their energy release is mass-dependent but steeper than the negative IMF slope, the energetic feedback retains a positive mass dependence, so that a filled IMF regulates star formation (SF) more strongly than truncated IMFs, though only stellar number fractions exist. The higher SFR of the truncated IMF in the simulation leads to more Type II supernovae (SNeII), driving galactic winds. Whether this results from the model-inherent larger SFR is questioned and therefore explored analytically. This shows the expected result for the Lyman continuum, but that the total SNII energy release is equal for both IMF modes, while the power is smaller for the truncated IMF. Reasonably, the different IMFs leave fingerprints in the abundance ratios of massive to intermediate-mass star elements.
... They conclude that an IMF which is deficient of massive stars in DGs and LSBs is consistent with their data because the Hα luminosity flattens at very low SFRs (Pflamm-Altenburg et al. 2007). This Hα-to-FUV flux divergence at low SFR is also documented by Meurer et al. (2009) as is the decreasing ratio of stellar ionizing to non-ionizing flux with smaller galaxy luminosity and mass, as well as a lower SFR (Meurer et al. 2009;Boselli et al. 2009). Calzetti et al. (2010) find that a tendency towards a decreasing Hα luminosity-to-cluster mass M cl exists, but not as strong as expected from ii). ...
... They conclude that an IMF which is deficient of massive stars in DGs and LSBs is consistent with their data because the Hα luminosity flattens at very low SFRs (Pflamm-Altenburg et al. 2007). This Hα-to-FUV flux divergence at low SFR is also documented by Meurer et al. (2009) as is the decreasing ratio of stellar ionizing to non-ionizing flux with smaller galaxy luminosity and mass, as well as a lower SFR (Meurer et al. 2009;Boselli et al. 2009). Calzetti et al. (2010) find that a tendency towards a decreasing Hα luminosity-to-cluster mass M cl exists, but not as strong as expected from ii). ...
When star clusters are formed at low star-formation rates (SFRs), their stellar initial mass function (IMF) can hardly be filled continuously with stars at each mass. This lack holds for massive stars and is observationally verified by the correlation between star-cluster mass and its most massive cluster star. Since galaxy evolution is strongly affected by massive stars, numerical models should account for this lack. Because a filled IMF is mostly applied even when only fractions of massive stars form, here we investigate by 3D chemo-dynamical simulations of isolated dwarf galaxies how deviations from a standard IMF in star clusters affect the evolution. We compare two different IMF recipes, a filled IMF with one truncated at a maximum mass at which a single complete star forms. Attention is given to energetic and chemical feedback by massive stars. Since their energy release is mass dependent but steeper than the negative IMF slope, the energetic feedback retains a positive mass dependence, so that a filled IMF regulates SF stronger than truncated IMFs, though only stellar number fractions exist. The higher SFR of the truncated IMF in the simulation leads to more supernovae II (SNeII), driving galactic winds. Whether this results from the model-inherent larger SFR is questioned and therefore analytically explored. This shows the expected result for Lyman continuum, but that the total SNII energy release is equal for both IMF modes, while the power is smaller for the truncated IMF. Reasonably, the different IMFs leave fingerprints in abundance ratios of massive-to-intermediate-mass star elements.
... In contrast, it is shown that an IMF whose slope varies, including intermediate-mass stars, is not in good agreement with the cosmic CCSN rate, owing to a resultant low cosmic star formation (Ziegler et al. 2022). We stress that our adopted IMF is also consistent with observational evidence for the variable flux ratio of Hα to UV emission, which trace the stars with M 20M e and 3M e , respectively, among galaxies whose SFRs are different (Lee et al. 2009;Meurer et al. 2009). However, we still need more efforts in the future to connect cosmic star formation to the CCSN rate in terms of nonuniversal IMFs to raise the accuracy of the DSNB flux prediction. ...
... However, we still need more efforts in the future to connect cosmic star formation to the CCSN rate in terms of nonuniversal IMFs to raise the accuracy of the DSNB flux prediction. In addition, we must keep in mind that there could be an uncertainty in the predicted frequency of BH formation against redshift; there are implications that the high-mass end of the IMF might correlate with the magnitude of the SFR (e.g., Lee et al. 2009;Meurer et al. 2009;Pflamm-Altenburg et al. 2009). ...
Fluxes of the diffuse supernova neutrino background (DSNB) are calculated based on a new modeling of galactic chemical evolution, where a variable stellar initial mass function (IMF), depending on the galaxy type, is introduced and black hole (BH) formation from the failed supernova is considered for progenitors heavier than 18 M ⊙ . The flux calculations are performed for different combinations of the star formation rate, nuclear equation of state, and neutrino mass hierarchy, to examine the systematic effects from these factors. In any case, our new model predicts the enhanced DSNB ν ¯ e flux at E ν ≳ 30 MeV and E ν ≲ 10 MeV, due to more frequent BH formation and a larger core-collapse rate at high redshifts in early-type galaxies, respectively. Event rate spectra of the DSNB ν ¯ e at a detector from the new model are shown, and the detectability at water-based Cherenkov detectors, Super-Kamiokande with a gadolinium dissolution and Hyper-Kamiokande, is discussed. In order to investigate the impacts of the assumptions in the new model, we prepare alternative models, based on different IMF forms and treatments of BH formation, and estimate the discrimination capabilities between the new and alternative models at these detectors.
... In contrast, it is shown that a IMF whose slope varies including intermediate-mass stars is not in good agreement with the cosmic CCSN rate owing to a resultant low cosmic star formation (Ziegler et al. 2022). We stress that our adopted IMF is also consistent with observational evidence for the variable flux ratio of Hα to UV emission which trace the stars with M > ∼ 20M ⊙ and > ∼ 3M ⊙ , respectively, among galaxies whose SFRs are different (Meurer et al. 2009;Lee et al. 2009). However, we still need more efforts in the future to connect cosmic star formation to the CCSN rate in terms of non-universal IMFs to raise accuracy of the DSNB flux prediction. ...
... However, we still need more efforts in the future to connect cosmic star formation to the CCSN rate in terms of non-universal IMFs to raise accuracy of the DSNB flux prediction. In addition, we must keep in mind that there could be an uncertainty in the predicted frequency of BH formation against redshift; there are implications that the highmass end of IMF might correlate with the magnitude of SFR (e.g., Meurer et al. 2009;Lee et al. 2009;Pflamm-Altenburg et al. 2009). ...
Fluxes of the diffuse supernova neutrino background (DSNB) are calculated based on a new modeling of galactic chemical evolution, where a variable stellar initial mass function (IMF) depending on the galaxy type is introduced and black hole (BH) formation from the failed supernova is considered for progenitors heavier than 18$M_{\odot}$. The flux calculations are performed for different combinations of the star formation rate, nuclear equation of state, and neutrino mass hierarchy to examine the systematic effects from these factors. In any case, our new model predicts the enhanced DSNB $\bar{\nu}_{e}$ flux at $E_\nu \gtsim 30$~MeV and $E_\nu \ltsim 10$~MeV due to more frequent BH formation and a larger core collapse rate at high redshifts in the early-type galaxies, respectively. Event rate spectra of the DSNB $\bar{\nu}_{e}$ at a detector from the new model are shown and detectability at water-based Cherenkov detectors, SK-Gd and Hyper-Kamiokande, is discussed. In order to investigate impacts of the assumptions in the new model, we prepare alternative models based on the different IMF form and treatment of BH formation, and estimate discrimination capabilities between the new and alternative models at these detectors.
... One possibility is to use the H α equivalent width EW(H α) (Dottori 1981;Copetti et al. 1986;Fernandes et al. 2003;Levesque & Leitherer 2013) or the H α/FUV ratio. Both fluxes are extensively used, most commonly as tracers for star formation (Lee et al. 2009) or to constrain the initial mass function (Meurer et al. 2009;Hermanowicz et al. 2013), but they have also been used on cloud scales as age indicators (e.g. Sánchez-Gil et al. 2011;Faesi et al. 2014). ...
Connecting the gas in HII regions to the underlying source of the ionizing radiation can help us constrain the physical processes of stellar feedback and how HII regions evolve over time. With PHANGS$\unicode{x2013}$MUSE we detect nearly 24,000 HII regions across 19 galaxies and measure the physical properties of the ionized gas (e.g. metallicity, ionization parameter, density). We use catalogues of multi-scale stellar associations from PHANGS$\unicode{x2013}$HST to obtain constraints on the age of the ionizing sources. We construct a matched catalogue of 4,177 HII regions that are clearly linked to a single ionizing association. A weak anti-correlation is observed between the association ages and the H$\alpha$ equivalent width EW(H$\alpha$), the H$\alpha$/FUV flux ratio and the ionization parameter, log q. As all three are expected to decrease as the stellar population ages, this could indicate that we observe an evolutionary sequence. This interpretation is further supported by correlations between all three properties. Interpreting these as evolutionary tracers, we find younger nebulae to be more attenuated by dust and closer to giant molecular clouds, in line with recent models of feedback-regulated star formation. We also observe strong correlations with the local metallicity variations and all three proposed age tracers, suggestive of star formation preferentially occurring in locations of locally enhanced metallicity. Overall, EW(H$\alpha$) and log q show the most consistent trends and appear to be most reliable tracers for the age of an HII region.
