Environments of active galactic nuclei at z < 1.5 in the UKIDSS Ultra-Deep Survey

Abstract

We investigate the environments of both X-ray and radio-loud active galactic nuclei (AGN) within the UK Infrared Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Ultra-deep Survey (UDS) using deep infrared selection to sample the galaxy density field in the redshift range 1.0 ≤z≤ 1.5. Using angular cross-correlation techniques we find that both X-ray and radio-loud AGN preferentially reside in overdense environments. We also find that both types of AGN cluster more strongly with those galaxies classified as 'passive' rather than those that are actively star forming. We infer clustering scalelengths comparable to those of passive red galaxies, suggesting that typical AGN at these epochs reside in dark matter haloes of mass M≳ 1013M⊙. A closer look at the small-scale environments of the AGN reveals that the neighbouring galaxies of radio-loud AGN have U-B colours more skewed towards the 'green valley' and the red sequence, whereas the neighbours of X-ray AGN show no difference to the general galaxy population. This suggests that although both AGN types live in overdense environments, the radio-loud AGN may be preferentially located in more evolved cluster cores, in a similar environment to low-powered radio AGN in the local Universe. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.