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

Shanks, T and Ansarinejad, B and Bielby, R M and Heywood, I and Metcalfe, N and Wang, L (2021) 'The nature of sub-millimetre galaxies I: a comparison of AGN and star-forming galaxy SED fits.', Monthly Notices of the Royal Astronomical Society, 505 (1). pp. 1509-1529.


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 per cent of such sourcesbrighter than ≈3 mJy host quasars. Here, we analyse a complete sample of 12 sub-mm LABOCA/ALMA 870-μm sources in the centre of the William Herschel Deep Field (WHDF) with multiwavelength 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 SEDs 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 quasar model, 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.

Item Type:Article
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Publisher statement:© 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society This is an Open Access article distributed under the terms of the Creative Commons Attribution License (, which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
Date accepted:25 April 2021
Date deposited:19 August 2021
Date of first online publication:06 May 2021
Date first made open access:19 August 2021

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