The Evolving Interstellar Medium of Star-Forming Galaxies since Z = 2 as Probed by their Infrared Spectral Energy Distributions
Résumé
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$^{12}$ $L_\odot$). 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, $\langle U \rangle$, 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 ($M_*$/$M_\odot$) ≥ 9.7 we measure this quantity, $\langle U \rangle$, 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 $\langle U \rangle$ (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, $\langle U \rangle$.
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