The contribution of AGN to the sub-mm population

Abstract

We find that X-ray sources in the Extended Chandra Deep Field South are strongly spatially correlated with LABOCA 870 micron 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 (L_X>10^44 ergs/s) have greater 870um 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 \pm 0.1 Jy/deg^2 to the sub-mm background, ~3% of the total, in agreement with the prediction of an obscured 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% to a total of 25-40%, although in a unified model the AGN contribution would only reach ~13%, 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.

15 pages, 13 figures; submitted to MNRAS

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• An ALMA Survey of Submillimeter Galaxies in the Extended Chandra Deep Field South: The AGN Fraction and X-ray Properties of Submillimeter Galaxies
• X-ray Detections of Sub-millimetre Galaxies: Active Galactic Nuclei Versus Starburst Contribution
• ALMA resolves extended star formation in high-z AGN host galaxies
• Submillimetre observations of X-ray active galactic nuclei in the William Herschel Deep Field
• The nature of sub-millimetre galaxies I: A comparison of AGN and star-forming galaxy SED fits
• Sub-millimetre source identifications and the micro-Jansky source population at 8.4 GHz in the William Herschel Deep Field
• Extending PLE models into the mid-IR, far-IR & sub-mm
• The nature of sub-millimetre galaxies II: an ALMA comparison of SMG dust heating mechanisms