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