High Gas Surface Densities yet Low UV Attenuation in z 1 Disc Galaxies
Abstract
The gas in galaxies is both the fuel for star formation and a medium that attenuates the light of the young stars. We study the relations between UV attenuation, spectral slope, star formation rates, and molecular gas surface densities in a sample of 28 z1 and a reference sample of 32 z0 galaxies that are detected in CO, far-infrared, and rest frame UV. The samples are dominated by disc-like galaxies close to the main SFR--mass relation. We find that the location of the z1 galaxies on the IRX-β plane is correlated with their gas-depletion time-scale τdep and can predict τdep with a standard deviation of 0.16 dex. We use IRX-β to estimate the mean total gas column densities at the locations of star formation in the galaxies, and compare them to the mean molecular gas surface densities as measured from CO. We confirm previous results regarding high NH/AV in z1 galaxies. We estimate an increase in the gas filling factor by a factor of 4--6 from z0 to z1 and a corresponding increase of factor 3--2 in the mean column densities of the star forming clouds. After accounting for the filling factor, the z1 and the z0 samples exhibit similar attenuation properties. These indicate to similar porous geometries to the molecular clouds in star-forming disc galaxies at 0<z<1.
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