paper

The nature of sub-millimetre galaxies I: A comparison of AGN and star-forming galaxy SED fits

arXiv:2010.07934 · doi:10.1093/mnras/stab1226

Abstract

High redshift sub-millimetre galaxies (SMGs) are usually assumed to be powered by star-formation. However, it has been clear for some time that 20% of such sources brighter than mJy host quasars. Here we analyse a complete sample of 12 sub-mm LABOCA/ALMA 870 m sources in the centre of the William Herschel Deep Field (WHDF) with multi-wavelength data available from the X-ray to the radio bands. Previously, two sources were identified as X-ray absorbed quasars at and . By comparing their spectral energy distributions (SEDs) with unabsorbed quasars in the same field, we confirm that they are dust reddened although at a level significantly lower than implied by their X-ray absorption. Then we compare the SED's of all the sources to dust-reddened AGN and star-forming galaxy models. This optical/NIR comparison combined with Spitzer MIR colours and faint Chandra X-ray detections shows that 7/12 SMGs are best fitted with an obscured quasarmodel, a further 3/12 show no preference between AGN and star-forming templates, leaving only a spiral galaxy and one unidentified source. So in our complete sample, the majority (10/12) of bright SMGs are at least as likely to fit an AGN as a star-forming galaxy template, although no claim is made to rule out the latter as SMG power sources. We then suggest modifications to a previous SMG number count model and conclude that obscured AGN in SMGs may still provide the dominant contribution to both the hard X-ray and sub-millimetre backgrounds.

arXiv comment22 pages, 11 Figures and 6 Tables. MNRAS accepted

References in corpus (50)

Cited by in corpus (9)