A Herschel view of the far-infrared properties of submillimetre galaxies
Magnelli et al. (2010, 2012a)
Since their discovery in the early 90s, submillimetre galaxies (SMGs) have been the subject of many studies as their submm emission suggested that they were the most extreme starburst of our Universe. Thanks to these studies we knew that SMGs were situated at very high redshift, i.e., z~2 (Chapman et al. 2005), that they were relatively compact and massive systems (Tacconi et al. 2008) and that the most luminous of them were mainly associated with major merger events (Engel et al. 2010). However, before the advent of Herschel most of their infrared properties and consequently their extreme star-formation activities were inferred from large and uncertain extrapolation. Therefore, simulations that were unable to reproduce at the same time the number density and the extreme star-formation activity of SGMs were questioning the accuracy of such large extrapolation (e.g., Davé et al. 2010).
Using deep far-infrared observation from the Herschel space observatory I have studied for the first time the dust temperatures and infrared luminosities of a large sample of SMGs. My results unambiguously unveil the diversity of the SMG population. Some SMGs exhibit extreme infrared luminosities of ∼10^13 L⊙ and relatively warm dust components (~40K), while others are fainter (a few times 10^12 L⊙) and are biased towards cold dust temperatures (~20K). The extreme infrared luminosities of about half of our SMGs (LIR~10^12.7 L⊙) imply star formation rates (SFRs) of >500 M⊙ yr−1. 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, consistent with results from hydrodynamic simulations of major mergers (Hayward, [...], Magnelli et al. 2012). 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. Studying the future of such luminous SMGs in the context of the formation of local massive elliptical galaxies is essential (Toft, [...], Magnelli et al. 2014).
Dust temperature-luminosity relation inferred from our single temperature model. Red circles present the OFRG sample of Magnelli et al. (2010). The striped area presents results for SMGs extrapolated by Chapman et al. (2005) from radio and submm data. The Chapman et al. (2003) derivation of the median and interquartile range of the Tdust − LIR relation observed at z ∼ 0 is shown by solid and dashed-dotted lines, linearly extrapolated to 10^13 L⊙. The dashed line represent the dust temperature-luminosity relation derived in Roseboom et al. (2011) for mm-selected sample observed with SPIRE and assuming a single modified blackbody model.
(Left) Infrared luminosities as function of the submm flux density. Blue squares represent SMGs situated in blank fields while green
diamonds represent lensed-SMGs. OFRGs from Magnelli et al. (2010) are presented with left red arrows. The solid and dashed lines show the
linear fit to the LIR − S850 relation and the 1σ envelope (LIR[L⊙] = 10^11.33±0.29 × S850^1.59 [mJy]). Dotted lines show the LIR − S850 relation followed by 850
single modified (β = 1.5) blackbody functions at 20, 35 and 50 K. (Right) Infrared luminosities as function of the redshift. The symbols are same as in the left panel but OFRGs are represented by red filled circles. Blue dotted, red dashed and green dotted-dashed lines present the lower limit of the parameter space reachable using our deep radio (i.e., 20 μJy), PACS 160 μm (i.e., 3 mJy) and MIPS-24 μm (i.e., 20 μJy) observations of the GOODS-N field, respectively. Note that in these figures galaxies with high χ2 value do not lie in a particular region of these plots but are rather randomly distributed.