Submillimetre observations of X-ray active galactic nuclei in the William Herschel Deep Field

Abstract

We investigate the contribution made by active galactic nuclei (AGN) to the high-redshift, luminous, submillimetre (submm) source population using deep (< 2 mJy/beam) Large Apex Bolometer Camera (LABOCA) 870 um observations within the William Herschel Deep Field (WHDF). This submm data complements previously obtained Chandra X-ray data of the field, from which AGN have been identified with the aid of follow-up optical spectra. From the LABOCA data, we detect 11 submm sources (based on a detection threshold of 3.2 sigma) with estimated fluxes of > 3 mJy/beam. Of the 11 identified submm sources, we find that 2 coincide with observed AGN and that, based on their hardness ratios, both of these AGN appear to be heavily obscured. We perform a stacking of the submm data around the AGN, which we group by estimated column density, and find that only the obscured (N_H > 10^22 cm^2) AGN show significant associated submm emission. These observations support the previous findings of Page et al and Hill et al that obscured AGN preferentially show submm emission. Hill et al have argued that, in this case, the contribution to the observed submm emission (and thus the submm background) from AGN heating of the dust in these sources may be higher than previously thought.