Herschel unveils a puzzling uniformity of distant dusty galaxies
  The Herschel Space Observatory enables us to accurately measure the
bolometric output of starburst galaxies and active galactic nuclei (AGN) by
directly sampling the peak of their far-infrared (IR) emission. Here we examine
whether the spectral energy distribution (SED) and dust temperature of galaxies
have strongly evolved since z~2.5. We use Herschel deep extragalactic surveys
from 100 to 500um to compute total IR luminosities in galaxies down to the
faintest levels, using PACS and SPIRE in the GOODS-North field (PEP and HerMES
key programs). We show that measurements in the SPIRE bands can be used below
the statistical confusion limit if information at higher spatial resolution is
used to identify isolated galaxies whose flux is not boosted by bright
neighbors. Below z~1.5, mid-IR extrapolations are correct for star-forming
galaxies with a dispersion of only 40% (0.15dex), therefore similar to z~0
galaxies. This narrow distribution is puzzling when considering the range of
physical processes that could have affected the SED of these galaxies.
Extrapolations from only one of the 160um, 250um or 350um bands alone tend to
overestimate the total IR luminosity. This may be explained by the lack of
far-IR constraints around and above ~150um (rest-frame) on local templates. We
also note that the dust temperature of luminous IR galaxies around z~1 is
mildly colder by 10-15% than their local analogs and up to 20% for ULIRGs at
z~1.6. Above z=1.5, distant galaxies are found to exhibit a substantially
larger mid- over far-IR ratio, which could either result from stronger broad
emission lines or warm dust continuum heated by a hidden AGN. Two thirds of the
AGNs identified in the field with a measured redshift exhibit the same behavior
as purely star-forming galaxies. Hence a large fraction of AGNs harbor star
formation at very high SFR and in conditions similar to purely star-forming
galaxies.
