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 ∼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∼ 0 to bright Hyper‐LIRG thresholds at z∼ 2. The transition to dominance by obscured AGN evolves similarly, at factor of several higher LIR. At all redshifts, non‐merging systems dominate the total luminosity/SFR density, with merger‐induced bursts constituting ∼5–10 per cent and AGN ∼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.

Dusty star-forming galaxies at high redshift

“…Theoretical models aiming to quantify what fraction of galaxies at a given luminosity are galaxy mergers versus disks produce a wide range of answers (e.g. Baugh et al, 2005;Davé et al, 2010;Dekel et al, 2009a;Narayanan et al, 2010a;Hopkins et al, 2010;González et al, 2011;Hayward et al, 2013b), and any observational constraints in this area are quite valuable (we discuss this issue in more detail in § 10).…”

“…Hopkins et al, 2010;Hayward et al, 2013b) have developed techniques where they combine duty cycles of particular events with galaxy merger rates and mass functions derived from cosmological dark matter-only simulations. As an illustrative example, Hayward et al (2013b) inferred the number counts of 850µm-selected SMGs by running a large suite of idealized disk galaxies and mergers over a large range of galaxy masses and merger mass ratios through dust radiative transfer calculations.…”

“…This dilemma originates in the fact that there are not enough mergers in the simulations for all SMGs to be accounted for by mergers, at least for galaxies as massive as M * ∼ 10 11 M (e.g. Guo & White, 2008;Hopkins et al, 2010). Without bursts, the SFR is, to first order, regulated by the gas accretion rate from the IGM, which is dictated by the ever-growing gravitational potential in the halo (e.g.…”

“…The hybrid methodology to extrapolate the properties of idealized simulations to a cosmological context was first developed by Hopkins et al (2010). The basic construct behind these models was to utilize theoretical halo mass functions, and assign galaxies to them utilizing an abundance matching technique (e.g.…”

“…These would then be combined with the results of high-resolution merger calculations in order to determine the typical burst luminosity from a galaxy merger, or steady-state SFR from an isolated disk galaxy of a given mass. By convolving the two, Hopkins et al (2010) measured the contribution of mergers, AGN and isolated-disk galaxies to observed infrared luminosity functions.…”