<i>Herschel</i>unveils a puzzling uniformity of distant dusty galaxies

Abstract

The <i>Herschel<i/> 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 star-formation rates (SFR) and any evidence for a major change in their star-formation properties. We use <i>Herschel<i/> deep extragalactic surveys from 100 to 500 <i>μ<i/>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 <i>μ<i/>m prior positions. We show that measurements in the SPIRE bands can be used below the <i>statistical<i/> confusion limit if information at higher spatial resolution is used, e.g. at 24 <i>μ<i/>m, to identify “isolated” galaxies whose flux is not boosted by bright neighbors. Below <i>z<i/> ~ 1.5, mid-IR extrapolations are correct for star-forming galaxies with a dispersion of only 40% (0.15 dex), therefore similar to <i>z<i/> ~ 0 galaxies, over three decades in luminosity below the regime of ultra-luminous IR galaxies (ULIRGs, <i>L<i/><sub>IR<sub/> <i>≥<i/> 10<sup>12<sup/> ). 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 <i>μ<i/>m, 250 <i>μ<i/>m or 350 <i>μ<i/>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 <i>μ<i/>m (rest-frame) before <i>Herschel<i/> on those templates. We also note that the dust temperature of luminous IR galaxies (LIRGs, <i>L<i/><sub>IR<sub/> <i>≥<i/> 10<sup>11<sup/> ) around <i>z<i/> ~ 1 is mildly colder by 10–15% than their local analogs and up to 20% for ULIRGs at <i>z<i/> ~ 1.6 (using a single modified blackbody-fit to the peak far-IR emission with an emissivity index of <i>β<i/> = 1.5). Above <i>z<i/> = 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.