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 over the last 80% of the age of the Universe. We discuss possible consequences for the determination of starformation rates (SFR) and any evidence for a major change in their star-formation properties. We use Herschel deep extragalactic surveys from 100 to 500 μ m 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). An extension to fainter luminosities is done by stacking images on 24 μ m prior positions. We show that measurements in the SPIRE bands can be used below the statistical confusion limit if information at higher spatial resolution is used, e.g. at 24 μ m, 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% (0.15 dex), therefore similar to z ∼ 0 galaxies, over three decades in luminosity below the regime of ultra-luminous IR galaxies (ULIRGs, L_IR ≥ 10¹² L_⊙). 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 160 μ m, 250 μ m or 350 μ m bands alone tend to overestimate the total IR luminosity. This may be explained by the lack of far-IR constraints around and above ∼150 μ m (rest-frame) before Herschel on those templates. We also note that the dust temperature of luminous IR galaxies (LIRGs, L_IR ≥ 10¹¹ L_⊙) around z ∼ 1 is mildly colder by 10-15% than their local analogs and up to 20% for ULIRGs at z ∼ 1.6 (using a single modified blackbody-fit to the peak far-IR emission with an emissivity index of β = 1.5). 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 coeval star formation at very high SFR and in conditions similar to purely star-forming galaxies.