THE EVOLVING INTERSTELLAR MEDIUM OF STAR-FORMING GALAXIES SINCE z=2 AS PROBED BY THEIR INFRARED SPECTRAL ENERGY DISTRIBUTIONS

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 (M-dust) for main-sequence (MS) galaxies, which obey a tight correlation between star formation rate (SFR) and stellar mass (M-*), and for starbursting galaxies that fall outside that relation. Exploiting the correlation of gas-to-dust mass with metallicity (M-gas/M-dust-Z), we use our measurements to constrain the gas content, CO-to-H-2 conversion factors (alpha(CO)), and star formation efficiencies (SFE) of these distant galaxies. Using large statistical samples, we confirm that alpha(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 (L-IR > 10(12) L-circle dot). For galaxies within the MS, we show that the variations of specific star formation rates (sSFRs = 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 (L-IR/M-dust) and the primary parameter defining the shape of the IR spectral energy distribution (SED), is equivalent to SFE/Z. For MS galaxies with stellar mass log(M-*/M-circle dot) >= 9.7 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 M-gas-SFR. Instead, we show that < U > (or equally L-IR/M-dust) does evolve, with MS galaxies having harder radiation fields and thus warmer temperatures as redshift increases from z = 0 to 2, a trend that 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 that are independent of their sSFR or M* but vary as a function of redshift with only one parameter, < U >.