Mergers, AGN, and 'Normal' Galaxies: Contributions to the Distribution
  of Star Formation Rates and Infrared Luminosity Functions
  We use a novel method to predict the contribution of normal star-forming
galaxies, merger-induced bursts, and obscured AGN, to IR luminosity functions
(LFs) and global SFR densities. We use empirical halo occupation constraints to
populate halos 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=0-6.
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 ~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' between normal and
merger-dominated sources increases from the LIRG-ULIRG threshold at z~0 to
bright Hyper-LIRG thresholds at z~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 ~5-10% and AGN ~1-5%. 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.
