2006MNRAS.370.1159B

We construct rest-frame luminosity functions (LFs) at 3.6, 4.5, 5.8, 8 and 24 µm 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-µm galaxy LFs show evidence for moderate positive luminosity evolution up to z ∼ 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 ∼ 0.5-1 but flattening, or even declining, at higher redshift. Conversely, the 24-µm 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 ∝ (1 + z)^γ with γ = 4.5^+0.7_–0.6 and the majority of this evolution occurring since z ∼ 1. Optical QSOs, however, show positive luminosity evolution in all bands, out to the highest redshifts (3 < z < 4). Modelling as L* ∝ (1 + z)^γ gave γ = 1.3^+0.1_–0.1 at 3.6µm,γ = 1.0^+0.1_–0.1 at 4.5µm and stronger evolution at the longer wavelengths (5.8, 8 and 24µm), of γ ∼ 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.