Luminosity functions for galaxies and quasars in the Spitzer wide-area infrared extragalactic legacy survey

We construct rest-frame luminosity functions (LFs) at 3.6, 4.5, 5.8, 8 and 24 mu m over the redshift range 0 < z < 2 for galaxies and 0 < z < 4 for optical quasi-stellar objects (QSOs), using optical and infrared (IR) data from the Spitzer Wide-area Infrared Extragalactic (SWIRE) Survey. The 3.6- and 4.5-mu m galaxy LFs show evidence for moderate positive luminosity evolution up to z similar to 1.5, consistent with the passive ageing of evolved stellar populations. Their comoving luminosity density was found to evolve passively, gradually increasing out to z similar to 0.5 - 1 but flattening, or even declining, at higher redshift. Conversely, the 24-mu m galaxy LF, which is more sensitive to obscured star formation and/or active galactic nuclei (AGN) activity, undergoes strong positive evolution, with the derived IR energy density and star formation rate (SFR) density proportional to (1 + z)(gamma) with gamma = 4.5(-0.6)(+0.7) and the majority of this evolution occurring since z similar to 1. Optical QSOs, however, show positive luminosity evolution in all bands, out to the highest redshifts (3 < z < 4). Modelling as L* proportional to ( 1 + z)(gamma) gave gamma = 1.3(-0.1)(+0.1) at 3.6 mu m, gamma = 1.0(-0.1)(+0.1) at 4.5 mu m and stronger evolution at the longer wavelengths (5.8, 8 and 24 mu m), of gamma similar to 3. Comparison of the galaxy LFs to predictions from a semi-analytic model based on cold dark matter (CDM) indicates that an initial mass function (IMF) skewed towards higher mass star formation in bursts compared to locally be preferred. As a result, the currently inferred massive SFRs in distant submm sources may require substantial downwards revision.