The Evolving Interstellar Medium of Star Forming Galaxies Since z=2 as Probed by Their Infrared Spectral Energy Distributions

Abstract: Using data from the mid-infrared to millimeter wavelengths for individual galaxies and for stacked ensembles at 0.5<z\<2, we derive robust estimates of dust masses (Mdust) for main sequence (MS) galaxies, which obey a tight correlation between star formation rate (SFR) and stellar mass (M*), and for star-bursting galaxies that fall outside that relation. Exploiting the correlation of gas to dust mass with metallicity (Mgas/Mdust -Z), we use our measurements to constrain the gas content, CO-to-H2 conversion factors (a_co) and star formation efficiencies (SFE) of these distant galaxies. Using large statistical samples, we confirm that a_co and SFE are an order of magnitude higher and lower, respectively, in MS galaxies at high redshifts compared to the values of local galaxies with equivalently high infrared luminosities. For galaxies within the MS, we show that the variations of specific star formation rates (sSFR=SFR/M*) are driven by varying gas fractions. For relatively massive galaxies like those in our samples, we show that the hardness of the radiation field, <U>, which is proportional to the dust mass weighted luminosity (LIR/Mdust), and the primary parameter defining the shape of the SED, is equivalent to SFE/Z. For MS galaxies we measure this quantity, <U>, showing that it does not depend significantly on either the stellar mass or the sSFR. This is explained as a simple consequence of the existing correlations between SFR-M*, M*-Z and Mgas-SFR. Instead, we show that <U> (or LIR/Mdust) does evolve, with MS galaxies having harder radiation fields and thus warmer temperatures as redshift increases from z=0 to 2, a trend which can also be understood based on the redshift evolution of the M*-Z and SFR-M* relations. These results motivate the construction of a universal set of SED templates for MS galaxies which vary as a function of redshift with only one parameter, <U>.

• The paper provides robust gas and dust mass estimates from infrared SEDs, revealing a decrease in the gas-to-dust ratio with increasing metallicity in main sequence galaxies.
• The paper demonstrates that starburst galaxies exhibit higher star formation efficiencies than main sequence galaxies, likely driven by major mergers.
• The paper shows that variations along the main sequence are primarily due to changes in gas fractions and evolving dust temperatures with redshift.

Overview of "The Evolving Interstellar Medium of Star Forming Galaxies since z=2 as Probed by Their Infrared Spectral Energy Distributions"

The research conducted by Magdis et al. provides a comprehensive investigation into the properties of the interstellar medium (ISM) of star-forming galaxies across a significant fraction of cosmic history, specifically from redshift $z \sim 2$ to the present. Through the examination of infrared spectral energy distributions (SEDs) from a variety of galaxies, the study seeks to elucidate the evolution of dust and gas content, as well as the star formation efficiencies (SFE) in these galaxies.

Key Findings

1. Gas and Dust Estimations: The study utilizes infrared to millimeter wavelength data to derive dust mass estimates, which are then used to infer gas content in both main sequence (MS) and non-Main Sequence or starburst galaxies. For the MS galaxies, the research confirms that the gas mass to dust mass ratio decreases with increasing metallicity.
2. Star Formation Efficiencies: A significant finding of the study is the observed difference in star formation efficiencies between MS and starburst galaxies. While starburst galaxies exhibit a more intense phase of star formation, likely due to major mergers, MS galaxies show lower SFEs despite their high infrared luminosities.
3. Main Sequence Dynamics: The research highlights that variations within the MS are primarily driven by changes in gas fractions rather than SFEs. The study underscores the importance of gas fraction as a determinant of the star formation rate (SFR) rather than variations in the efficiency of using the available gas for star formation.
4. Redshift Evolution: The study identifies a redshift-dependent increase in the hardness of the radiation field for MS galaxies. This evolution is mirrored by observed shifts in their infrared SED shapes, indicating changes in the dust temperature, which seem to be intrinsically tied to the cosmic evolution of metallicity.
5. Template SEDs: The results provide motivation for creating a new set of SED templates for MS galaxies that are parameterized as a function of redshift. This approach is significant as it offers a nuanced understanding of the ISM properties over cosmic time scales, aiding predictions of galaxy behavior at different epochs.

Theoretical and Practical Implications

• Theoretical Astrophysics: The findings directly contribute to our understanding of galaxy evolution, particularly regarding how star formation processes change over time and under varying environmental conditions. It challenges and complements existing models by providing empirical data on how SFEs and gas content evolve.
• Metallicity Dependency: The results further emphasize the tight correlation between metallicity and gas-to-dust ratios, which provides insights into the cycle of baryonic matter in galaxies.
• Applications in Observational Astronomy: By offering refined SED templates, the study enhances the accuracy of interpreting high-redshift galaxy observations, especially in the context of future missions and telescopes that will probe deeper into the Universe's history.

Future Directions

The conclusions drawn from this study open several avenues for future research. With the advent of facilities like the James Webb Space Telescope (JWST) and the Atacama Large Millimeter/submillimeter Array (ALMA), more precise and expansive datasets will become available. This development will help to validate and extend the current findings, particularly concerning the interplay of gas content and metallicity across different galaxy populations.

Furthermore, a more detailed exploration of starburst galaxies at high redshifts could elucidate the triggers and consequences of such intense star-forming episodes, offering a more complete picture of galaxy formation and evolution. Additionally, expanding the study to include more varied galaxy morphologies and environments, including quiescent and dwarf galaxies, could provide a fuller understanding of the ISM across the full spectrum of galactic systems.