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

Abstract

Using data from the mid-infrared to millimeter wavelengths for individual\ngalaxies and for stacked ensembles at 0.5&lt;z&lt;2, we derive robust estimates of\ndust masses (Mdust) for main sequence (MS) galaxies, which obey a tight\ncorrelation between star formation rate (SFR) and stellar mass (M*), and for\nstar-bursting galaxies that fall outside that relation. Exploiting the\ncorrelation of gas to dust mass with metallicity (Mgas/Mdust -Z), we use our\nmeasurements to constrain the gas content, CO-to-H2 conversion factors (a_co)\nand star formation efficiencies (SFE) of these distant galaxies. Using large\nstatistical samples, we confirm that a_co and SFE are an order of magnitude\nhigher and lower, respectively, in MS galaxies at high redshifts compared to\nthe values of local galaxies with equivalently high infrared luminosities. For\ngalaxies within the MS, we show that the variations of specific star formation\nrates (sSFR=SFR/M*) are driven by varying gas fractions. For relatively massive\ngalaxies like those in our samples, we show that the hardness of the radiation\nfield, &lt;U&gt;, which is proportional to the dust mass weighted luminosity\n(LIR/Mdust), and the primary parameter defining the shape of the SED, is\nequivalent to SFE/Z. For MS galaxies we measure this quantity, &lt;U&gt;, showing\nthat it does not depend significantly on either the stellar mass or the sSFR.\nThis is explained as a simple consequence of the existing correlations between\nSFR-M*, M*-Z and Mgas-SFR. Instead, we show that &lt;U&gt; (or LIR/Mdust) does\nevolve, with MS galaxies having harder radiation fields and thus warmer\ntemperatures as redshift increases from z=0 to 2, a trend which can also be\nunderstood based on the redshift evolution of the M*-Z and SFR-M* relations.\nThese results motivate the construction of a universal set of SED templates for\nMS galaxies which vary as a function of redshift with only one parameter, &lt;U&gt;.\n