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

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 $\approx 3$mJy host quasars. Here we analyse a complete sample of 12 sub-mm LABOCA/ALMA 870 $\mu$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 $z=1.32$ and $z=2.12$. 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 $z=0.046$ 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.

22 pages, 11 Figures and 6 Tables. MNRAS accepted

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• Lightning: An X-ray to Submillimeter Galaxy SED-Fitting Code With Physically-Motivated Stellar, Dust, and AGN Models
• Radio emission from dust-obscured galaxies
• The VST ATLAS Quasar Survey I: Catalogue
• Improved SED-Fitting Assumptions Result in Inside-Out Quenching at $z\sim 0.5$ and Quenching at All Radii Simultaneously at $z\sim 1$
• Detection of radio-AGN in dust-obscured galaxies using deep uGMRT radio continuum observations
• The nature of sub-millimetre galaxies II: an ALMA comparison of SMG dust heating mechanisms