The Contribution of Starbursts and Normal Galaxies to Infrared Luminosity Functions at z < 2

We present a parameterless approach to predict the shape of the infrared (IR) luminosity function (LF) at redshifts z <= 2. It requires no tuning and relies on only three observables: (1) the redshift evolution of the stellar mass function for star-forming galaxies, (2) the evolution of the specific star formation rate (sSFR) of main-sequence galaxies, and (3) the double-Gaussian decomposition of the sSFR distribution at fixed stellar mass into a contribution (assumed redshift- and mass-invariant) from main-sequence and starburst activity. This self-consistent and simple framework provides a powerful tool for predicting cosmological observables: observed IR LFs are successfully matched at all z <= 2, suggesting a constant or only weakly redshift-dependent contribution (8%-14%) of starbursts to the SFR density. We separate the contributions of main-sequence and starburst activity to the global IR LF at all redshifts. The luminosity threshold above which the starburst component dominates the IR LF rises from log(L _IR/L _⊙) = 11.4 to 12.8 over 0 < z < 2, reflecting our assumed (1+z)^2.8-evolution of sSFR in main-sequence galaxies.