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

TL;DR

This study analyzes the spectral energy distributions of high-redshift sub-millimetre galaxies, revealing that most are likely powered by obscured active galactic nuclei rather than solely by star formation, impacting models of cosmic backgrounds.

Contribution

It provides the first comprehensive comparison of SMG SEDs with AGN and star-forming galaxy models, highlighting the significant AGN contribution in bright SMGs.

Findings

Majority of bright SMGs are best fitted with obscured quasar models.

Many SMGs show signs of dust-reddened AGN activity.

Obscured AGN in SMGs may dominate the X-ray and sub-millimetre backgrounds.

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 $\approx3$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 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.