Journal Article

The contribution of AGN to the submillimetre population

Michael D. Hill and Tom Shanks

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 410, issue 2, pages 762-774
Published in print January 2011 | ISSN: 0035-8711
Published online December 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.17477.x
The contribution of AGN to the submillimetre population

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We find that X-ray sources in the Extended Chandra Deep Field-South are strongly spatially correlated with Large APEX Bolometer Camera (LABOCA) 870 μm sources. We investigate the dependence of this correlation on X-ray flux, hardness ratio and column density, finding that specifically faint and absorbed X-ray sources are significant sub-mm emitters. In the X-ray source redshift subsample we confirm the previous result that higher luminosity sources (LX > 1044 erg s−1) have greater 870 μm fluxes but we also find that this subsample selects against absorbed sources, faint in X-ray flux. Overall, we find that X-ray sources contribute 1.5 ± 0.1 Jy deg−2 to the sub-mm background, ≈3 per cent of the total, in agreement with the prediction of an obscured active galactic nuclei (AGN) model which also gives a reasonable fit to the bright sub-mm source counts. This non-unified model also suggests that when Compton-thick, X-ray-undetected sources are included, then the fractional AGN contribution to the sub-mm background would rise from ≈3 per cent to a total of 25–40 per cent, although in a unified model the AGN contribution would only reach ≈13 per cent, because the sub-mm flux of the X-ray sources is then more representative of the whole AGN population. Measurements of the dependence of sub-mm flux on X-ray flux, luminosity and column density all agree well with the predictions of the non-unified AGN model. Heavily absorbed, X-ray-undetected AGN could explain the further cross-correlation we find between sub-mm sources and z > 0.5 red galaxies. We conclude that sub-mm galaxies may contain the long-sought absorbed AGN population needed to explain the X-ray background.

Keywords: galaxies: high-redshift; quasars: general; submillimetre: galaxies; X-rays: galaxies

Journal Article.  10133 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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