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

Shanks, T; Ansarinejad, B; Bielby, RM; Heywood, I; Metcalfe, N; Wang, L

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


T Shanks

B Ansarinejad

RM Bielby

I Heywood

L Wang


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.


Shanks, T., Ansarinejad, B., Bielby, R., Heywood, I., Metcalfe, N., & 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), 1509-1529.

Journal Article Type Article
Acceptance Date Apr 25, 2021
Online Publication Date May 6, 2021
Publication Date 2021-07
Deposit Date Aug 19, 2021
Publicly Available Date Aug 19, 2021
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 505
Issue 1
Article Number 1509-1529
Pages 1509-1529


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Copyright Statement
© 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society<br /> 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.

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