Prof. Dimitra Rigopoulou

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 (Mdust) 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 (Mgas/M dust-Z), we use our measurements to constrain the gas content, CO-to-H2 conversion factors (αCO), and star formation efficiencies (SFE) of these distant galaxies. Using large statistical samples, we confirm that αCO 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 (LIR > 1012 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/Mdust) 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/Mdust) 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〉. © 2012. The American Astronomical Society. All rights reserved.

Abstract:

We present observations of the CO[J = 3 → 2] emission toward two massive and infrared luminous Lyman break galaxies (LBGs) at z = 3.21 and z = 2.92, using the IRAM Plateau de Bure Interferometer, placing first constraints on the molecular gas masses (M gas) of non-lensed LBGs. Their overall properties are consistent with those of typical (main-sequence) galaxies at their redshifts, with specific star formation rates 1.6 and 2.2Gyr-1, despite their large infrared luminosities (L IR (2-3) × 1012L) derived from Herschel. With one plausible CO detection (spurious detection probability of 10-3) and one upper limit, we investigate the evolution of the molecular gas-to-stellar mass ratio (M gas/M *) with redshift. Our data suggest that the steep evolution of M gas/M * of normal galaxies up to z 2 is followed by a flattening at higher redshifts, providing supporting evidence for the existence of a plateau in the evolution of the specific star formation rate at z > 2.5. © 2012. The American Astronomical Society. All rights reserved.