Abstract
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.
Highlights
Understanding the global star formation history of the Universe remains an important unresolved goal in cosmology
For example, the total mass density observed in old stars and the implied star formation rate (SFR) density have been cited as possible evidence of a time-dependent stellar initial mass function (IMF); but, a rising contribution from obscured active galactic nuclei (AGN) at high redshifts could mimic this effect (Hopkins & Beacom 2006; Dave 2008)
We compare to observations of all quantities, where available, and find reasonable agreement but with some interesting apparent discrepancies at high redshifts. We show how these populations relate to the scatter in SFRs at fixed galaxy masses, and in a global sense to the total SFR density, the SFR density in mergers and the fraction of the inferred SFR density which might really be driven by obscured AGN activity
Summary
Understanding the global star formation history of the Universe remains an important unresolved goal in cosmology. These systems exhibit many of the traits more commonly associated with merger-driven starbursts, including morphological disturbances, and may be linked to the emergence of massive, quenched (non star-forming), compact ellipticals at times as early as z ∼ 2–4 (Papovich et al 2005; Tacconi et al 2006, 2008; Schinnerer et al 2008; Younger et al 2008b; Chapman et al 2009) Reproducing their abundance and luminosities remains a challenge for current models of galaxy formation (Baugh et al 2005; Swinbank et al 2008; Narayanan et al 2009b; Younger et al 2009a). For example, the total mass density observed in old stars and the implied SFR density have been cited as possible evidence of a time-dependent stellar initial mass function (IMF); but, a rising contribution from obscured AGN at high redshifts could mimic this effect (Hopkins & Beacom 2006; Dave 2008).
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