Luminosity functions for galaxies and quasars in the Spitzer Wide-Area Infrared Extra-galactic (SWIRE) Legacy survey
T.S.R. Babbedge, M. Rowan-Robinson, M. Vaccari, J. A. Surace, C.J. Lonsdale, D.L. Clement, D. Farrah, F. Fang, A. Franceschini, E. Gonzalez-Solares, E. Hatziminaoglou, C.G. Lacey, S. Oliver, N. Onyett, I. Perez-Fournon, M. Polletta, F. Pozzi, G. Rodighiero, D.L. Shupe, B. Siana, H.E. Smith
Abstract
We construct rest-frame luminosity functions at 3.6, 4.5, 5.8, 8 and 24 microns over the redshift range 0<z<2 for galaxies and 0<z<4 for optical QSOs, using optical and infrared data from the Spitzer Wide-area InfraRed Extragalactic survey. The 3.6 and 4.5 micron 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 micron galaxy LF, which is more sensitive to obscured star formation and/or AGN activity, undergoes strong positive evolution, with the derived IR energy density and SFR density proportional to (1+z)^gamma with gamma=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* proportional to (1+z)^gamma gave gamma=1.3^{+0.1}_{-0.1} at 3.6 micron, gamma=1.0^{+0.1}_{-0.1} at 4.5 micron and stronger evolution at the longer wavelengths (5.8, 8 and 24 micron), of gamma~3. Comparison of the galaxy LFs to predictions from a semi-analytic model based on CDM indicate that an IMF skewed towards higher mass star formation in bursts compared to locally is preferred. As a result the currently inferred massive star formation rates in distant sub-mm sources may require substantial downwards revision.