A universal relation of dust obscuration across cosmic time

Abstract

We investigate dust obscuration as parametrized by the infrared excess IRX ≡ L_IR/L_UV in relation to global galaxy properties, using a sample of ∼32 000 local star-forming galaxies (SFGs) selected from SDSS (Sloan Digital Sky Survey), GALEX (Galaxy Evolution Explorer), and WISE (Wide-field Infrared Survey Explorer). We show that IRX generally correlates with stellar mass (M_*), star formation rate, gas-phase metallicity (Z), infrared luminosity (L_IR), and the half-light radius (R_e). A weak correlation of IRX with axial ratio (b/a) is driven by the inclination and thus seen as a projection effect. By examining the tightness and the scatter of these correlations, we find that SFGs obey an empirical relation of the form IRX = 10^α (L_IR)^{β } R_e^{-γ } (b/a)^{-δ }, where the power-law indices all increase with metallicity. The best-fitting relation yields a scatter of ∼0.17 dex and no dependence on stellar mass. Moreover, this empirical relation also holds for distant SFGs out to z = 3 in a population-averaged sense, suggesting it to be universal over cosmic time. Our findings reveal that IRX approximately increases with L_IR/R_e^{[1.3 - 1.5]} instead of L_IR/R_e² (i.e. surface density). We speculate this may be due to differences in the spatial extent of stars versus star formation and/or complex star-dust geometries. We conclude that not stellar mass but IR luminosity, metallicity, and galaxy size are the key parameters jointly determining dust obscuration in SFGs.