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 (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>.
