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

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 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.

Citation

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. https://doi.org/10.1093/mnras/stab1226

• 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
• DOI: https://doi.org/10.1093/mnras/stab1226