The Herschel★ PEP/HerMES luminosity function – I. Probing the evolution of PACS selected Galaxies to z ≃ 4

Abstract

We exploit the deep and extended far infrared data sets (at 70, 100 and 160\num) of the Herschel GTO PACS Evolutionary Probe (PEP) Survey, in combination\nwith the HERschel Multi tiered Extragalactic Survey (HerMES) data at 250, 350\nand 500 um, to derive the evolution of the restframe 35 um, 60 um, 90 um, and\ntotal infrared (IR) luminosity functions (LFs) up to z~4. We detect very strong\nluminosity evolution for the total IR LF combined with a density evolution. In\nagreement with previous findings, the IR luminosity density increases steeply\nto z~1, then flattens between z~1 and z~3 to decrease at z greater than 3.\nGalaxies with different SEDs, masses and sSFRs evolve in very different ways\nand this large and deep statistical sample is the first one allowing us to\nseparately study the different evolutionary behaviours of the individual IR\npopulations contributing to the IR luminosity density. Galaxies occupying the\nwell established SFR/stellar mass main sequence (MS) are found to dominate both\nthe total IR LF and luminosity density at all redshifts, with the contribution\nfrom off MS sources (0.6 dex above MS) being nearly constant (~20% of the total\nIR luminosity density) and showing no significant signs of increase with\nincreasing z over the whole 0.8<z<2.2 range. Sources with mass in the 10<\nlog(M/Msun) <11 range are found to dominate the total IR LF, with more massive\ngalaxies prevailing at the bright end of the high-z LF. A two-fold evolutionary\nscheme for IR galaxies is envisaged: on the one hand, a starburst-dominated\nphase in which the SMBH grows and is obscured by dust, is followed by an AGN\ndominated phase, then evolving toward a local elliptical. On the other hand,\nmoderately starforming galaxies containing a low-luminosity AGN have various\nproperties suggesting they are good candidates for systems in a transition\nphase preceding the formation of steady spiral galaxies.\n