The nature of sub-millimetre galaxies I: A comparison of AGN and star-forming galaxy SED fits. (arXiv:2010.07934v1 [astro-ph.GA])

The nature of sub-millimetre galaxies I: A comparison of AGN and star-forming galaxy SED fits. (arXiv:2010.07934v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Shanks_T/0/1/0/all/0/1">T. Shanks</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Ansarinejad_B/0/1/0/all/0/1">B. Ansarinejad</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Bielby_R/0/1/0/all/0/1">R.M. Bielby</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Heywood_I/0/1/0/all/0/1">I. Heywood</a> (2), <a href="http://arxiv.org/find/astro-ph/1/au:+Metcalfe_N/0/1/0/all/0/1">N. Metcalfe</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Wang_L/0/1/0/all/0/1">L. Wang</a> (3) ((1) CEA, Physics Department, University of Durham, UK, (2) Astrophysics, Department of Physics, University of Oxford, UK, (3) SRON Netherlands Institute for Space Research, Groningen, NL)

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.

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.

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