Herschelunveils a puzzling uniformity of distant dusty galaxies

Abstract

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 μm 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 μm 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 μm, 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 ≥ 10 12 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 μm, 250 μm or 350 μm bands alone tend to overestimate the total IR luminosity. This may be explained by the lack of far-IR constraints around and above ∼150 μm (rest-frame) before Herschel on those templates. We also note that the dust temperature of luminous IR galaxies (LIRGs, L IR ≥ 10 11 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.

A Model Connecting Galaxy Masses, Star Formation Rates, and Dust Temperatures across Cosmic Time

“…This systematic offset may partly be due to the choice of modified blackbody models fitted by these authors and partly a result of how they selected sources. Indeed, other authors who performed stacking analyses but fitted different models or had different selection criteria, including Pascale et al (2009), Amblard et al (2010, and Elbaz et al (2010), reported higher dust temperatures in alignment with our results. We note that these latter two studies, which are based on galaxy samples selected by Herschel, may be biased in temperature.…”

No Clear Submillimeter Signature of Suppressed Star Formation Among X-Ray Luminous Active Galactic Nuclei

“…In the redshift range z = 1-3 investigated here, the observed 250 μm fluxes correspond to the peak of the SEDs in the rest frame. As such, the L 250 to L IR correction factors do not vary much as a function of SED shape when compared to using shorter wavelengths (see Figure 3 in Elbaz et al 2010). Indeed, when we compared all of the Chary & Elbaz (2001) SEDs, redshifted to z = 2, the conversion factors were consistent within ≈20%; we also repeated our analysis using the main-sequence SED from Elbaz et al (2011) for all of the bins and found no significant difference.…”

A New Star Formation Rate Calibration From Polycyclic Aromatic Hydrocarbon Emission Features and Application to High-Redshift Galaxies

“…A PREVIEW FOR JWST IR SEDs evolve with redshift, so comparisons must be over similar redshift ranges or must make the necessary adjustments (e.g., Hernán-Caballero et al 2009;Papovich et al 2009;Elbaz et al 2010;Finkelstein et al 2011;Kirkpatrick et al 2012;Rujopakarn et al 2013). We first discuss previous works on this subject; in a following subsection we present results for our demonstration sample of gravitationally lensed, high-redshift galaxies as a case study for galaxies with luminosities and redshifts to be probed with JWST.…”