Mergers, active galactic nuclei and ‘normal’ galaxies: contributions to the distribution of star formation rates and infrared luminosity functions

Abstract

We use a novel method to predict the contribution of normal star-forming galaxies, merger-induced bursts and obscured active galactic nuclei (AGN), to infrared luminosity functions (LFs) and global star formation rate (SFR) densities. We use empirical halo occupation constraints to populate haloes with galaxies and determine the distribution of normal and merging galaxies. Each system can then be associated with high-resolution hydrodynamic simulations. We predict the distribution of observed luminosities and SFRs, from different galaxy classes, as a function of redshift from z = 0 to 6. We provide fitting functions for the predicted LFs, quantify the uncertainties, and compare with observations. At all redshifts, 'normal' galaxies dominate the LF at moderate luminosities similar to L(*) (the 'knee'). Merger-induced bursts increasingly dominate at L L(*); at the most extreme luminosities, AGN are important. However, all populations increase in luminosity at higher redshifts, owing to increasing gas fractions. Thus, the 'transition luminosity' between normal and merger-dominated sources increases from the luminous infrared galaxy (LIRG)-ultraluminous infrared galaxy threshold at z similar to 0 to bright Hyper-LIRG thresholds at z similar to 2. The transition to dominance by obscured AGN evolves similarly, at factor of several higher L(IR). At all redshifts, non-merging systems dominate the total luminosity/SFR density, with merger-induced bursts constituting similar to 5-10 per cent and AGN similar to 1-5 per cent. Bursts contribute little to scatter in the SFR-stellar mass relation. In fact, many systems identified as 'ongoing' mergers will be forming stars in their 'normal' (non-burst) mode. Counting this as 'merger-induced' star formation leads to a stronger apparent redshift evolution in the contribution of mergers to the SFR density. We quantify how the evolution in LFs depends on evolution in galaxy gas fractions, merger rates, and possible evolution in the Schmidt-Kennicutt relation. We discuss areas where more detailed study, with full radiative transfer treatment of complex three-dimensional clumpy geometries in mixed AGN-star-forming systems, is necessary.