arXiv Analytics

Sign in

arXiv:2010.07934 [astro-ph.GA]AbstractReferencesReviewsResources

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

T. Shanks, B. Ansarinejad, R. M. Bielby, I. Heywood, N. Metcalfe, L. Wang

Published 2020-10-15Version 1

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 $\approx3$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 models and to similarly reddened star-forming galaxy models, one based on the median SMG fit to the large AS2UDS survey. 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 vast majority (10/12) of bright SMGs are at least as likely to fit an AGN as a star-forming galaxy template. We then modify the model of Hill & Shanks (2011) and conclude that obscured AGN in SMGs may still provide the dominant contribution to both the hard X-ray and sub-millimetre backgrounds.

Comments: 23 pages, 12 Figures and 6 Tables. MNRAS submitted
Categories: astro-ph.GA
Related articles: Most relevant | Search more
arXiv:1505.04607 [astro-ph.GA] (Published 2015-05-18)
nIFTy Cosmology: Comparison of Galaxy Formation Models
arXiv:1210.5951 [astro-ph.GA] (Published 2012-10-22)
Comparison of Two Epochs of the Zeeman Effect in the 44 GHz Class I methanol (CH3OH) maser line in OMC-2
arXiv:1206.4828 [astro-ph.GA] (Published 2012-06-21)
A comparison between the stellar and dynamical masses of six globular clusters