AHerschelview of the far-infrared properties of submillimetre galaxies

Abstract

We study a sample of 61 submillimetre galaxies (SMGs) selected from ground-based surveys, with known spectroscopic redshifts and observed with the Herschel Space Observatory as part of the PACS Evolutionary Probe (PEP) and the Herschel Multi-tiered Extragalactic Survey (HerMES) guaranteed time key programmes. Our study makes use of the broad far-infrared and submillimetre wavelength coverage (100−600 μm) only made possible by the combination of observations from the PACS and SPIRE instruments aboard the Herschel Space Observatory. Using a power-law temperature distribution model to derive infrared luminosities and dust temperatures, we measure a dust emissivity spectral index for SMGs of β = 2.0 ± 0.2. Our results unambiguously unveil the diversity of the SMG population. Some SMGs exhibit extreme infrared luminosities of ∼10 13 L and relatively warm dust components, while others are fainter (a few times 10 12 L ) and are biased towards cold dust temperatures. Although at z ∼ 2 classical SMGs (>5 mJy at 850 μm) have large infrared luminosities (∼10 13 L ), objects only selected on their submm flux densities (without any redshift informations) probe a large range in dust temperatures and infrared luminosities. The extreme infrared luminosities of some SMGs (L IR 10 12.7 L , 26/61 systems) imply star formation rates (SFRs) of >500 M yr −1 (assuming a Chabrier IMF and no dominant AGN contribution to the FIR luminosity). Such high SFRs are difficult to reconcile with a secular mode of star formation, and may instead correspond to a merger-driven stage in the evolution of these galaxies. Another observational argument in favour of this scenario is the presence of dust temperatures warmer than that of SMGs of lower luminosities (∼40 K as opposed to ∼25 K), consistent with observations of local ultraluminous infrared galaxies triggered by major mergers and with results from hydrodynamic simulations of major mergers combined with radiative transfer calculations. Moreover, we find that luminous SMGs are systematically offset from normal star-forming galaxies in the stellar mass-SFR plane, suggesting that they are undergoing starburst events with short duty cycles, compatible with the major merger scenario. On the other hand, a significant fraction of the low infrared luminosity SMGs have cold dust temperatures, are located close to the main sequence of star formation, and therefore might be evolving through a secular mode of star formation. However, the properties of this latter population, especially their dust temperature, should be treated with caution because at these luminosities SMGs are not a representative sample of the entire star-forming galaxy population.

Star formation inz> 1 3CR host galaxies as seen byHerschel

“…Radio galaxies and quasars span the same range in cold dust temperatures, This range is similar to that obtained for z > 5 quasars (Leipski et al 2014) and to that estimated for distant submm galaxies (e.g. Magnelli et al 2012). Similarly to Leipski et al (2013), the inclusion of the 1300 K hot dust component in the SED fitting lowers the estimates of the cold dust temperatures by ∼5 K. By including the hot dust component, we preferentially select torus models that emit more of their energy at longer wavelengths.…”

“…Converting the star formation luminosities into SFRs using the calibration derived by Kennicutt (1998) gives 100 M yr −1 < SFR < 1000 M yr −1 , consistent with SFRs obtained for typical submm galaxies (SMGs) at comparable redshifts (e.g. Magnelli et al 2012).…”

TheHerschelVirgo Cluster Survey

“…Most galaxies follow a main sequence with a tight correlation between stellar mass and star formation, independent of luminosity and redshift. A smaller galaxy population, less than 20%, shows a starburst phase, with high specific star-formation rate (e.g., Magnelli et al 2012). Supported by these results, Béthermin et al (2012b) proposed a model that considers the AGN contribution, gravitational lensing, and assumed only two types of galaxy SEDs for the main sequence and the starburst population.…”

Hidden starbursts and active galactic nuclei at 0 < z < 4 from theHerschel-VVDS-CFHTLS-D1 field: Inferences on coevolution and feedback

“…The past averaged SFR was proxied by the M * -SFR infrared main sequence (MS), and therefore a galaxy significantly departing from the MS at a given redshift is considered a starburst galaxy (more specifically, when the ratio of the SSFR of a galaxy and a typical value of the SSFR for a MS galaxy at the same redshift exceeds two). This definition of starburst, which has been heavily used in the literature (see, e.g., Magnelli et al 2012 and references therein), relies heavily both on the premise that the infrared MS exists and that the typical value of the SSFR for an MS galaxy can be measured precisely as a function of redshift. The latter relies on both a precise quantification of the relationship between M * and SFR, the precision with which each parameter can be estimated, and the assumption that this relationship does not change based on the selection method of the sample nor the dynamic range of SFRs or stellar masses probed in a given sample.…”