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 ( 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 (α CO ), and star formation efficiencies (SFE) of these distant galaxies. Using large statistical samples, we confirm that α 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 ☉ ). 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 ☉ ) ⩾ 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 〉.