AGN environments at z<1.5 in the UKIDSS Ultra-Deep Survey

TL;DR

This study examines the environments of X-ray and radio-loud AGN at redshifts 1.0 to 1.5, revealing they prefer overdense regions and are associated with passive galaxies, indicating they reside in massive dark matter halos.

Contribution

It provides new insights into the clustering and environmental preferences of AGN at intermediate redshifts using deep infrared data and cross-correlation techniques.

Findings

Both AGN types reside in overdense environments.

AGN cluster more with passive galaxies than star-forming ones.

Radio-loud AGN are found in more evolved cluster cores.

Abstract

We investigate the environments of both X-ray and radio-loud AGN within the 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 scale lengths comparable to those of passive red galaxies, suggesting that typical AGN at these epochs reside in dark-matter halos of mass M >~ 10^13 M_sun. 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.