AEGIS: The Clustering of X-ray AGN Relative to Galaxies at z~1

TL;DR

This study measures the clustering of X-ray AGN at z~1 and finds they are more clustered than star-forming galaxies, especially in red host galaxies, indicating they reside in more massive halos and are linked to galaxy evolution.

Contribution

It provides the first detailed clustering analysis of non-quasar X-ray AGN at z~1, revealing their dependence on host galaxy color and comparison with optically-selected quasars.

Findings

X-ray AGN have a clustering scale length of ~6 h^-1 Mpc.

AGN in red hosts are more clustered than in blue hosts.

X-ray AGN are more clustered than optically-selected quasars at similar redshifts.

Abstract

We measure the clustering of non-quasar X-ray AGN at z=0.7-1.4 in the AEGIS field. Using the cross-correlation of 113 Chandra-selected AGN, with a median log L_X=42.8 erg s^-1, with ~5,000 DEEP2 galaxies, we find that the X-ray AGN are fit by a power law with a clustering scale length of r_0=5.95 +/-0.90 h^-1 Mpc and slope gamma=1.66 +/-0.22. X-ray AGN have a similar clustering amplitude as red, quiescent and `green' transition galaxies at z~1 and are significantly more clustered than blue, star-forming galaxies. The X-ray AGN clustering strength is primarily determined by the host galaxy color; AGN in red host galaxies are significantly more clustered than AGN in blue host galaxies, with a relative bias that is similar to that of red to blue DEEP2 galaxies. We detect no dependence of clustering on optical brightness, X-ray luminosity, or hardness ratio within the ranges probed here. We find evidence for galaxies hosting X-ray AGN to be more clustered than a sample of galaxies with matching joint optical color and magnitude distributions. This implies that galaxies hosting X-ray AGN are more likely to reside in groups and more massive dark matter halos than galaxies of the same color and luminosity without an X-ray AGN. In comparison to optically-selected quasars in the DEEP2 fields, we find that X-ray AGN at z~1 are more clustered than optically-selected quasars (with a 2.6-sigma significance) and therefore likely reside in more massive dark matter halos. Our results are consistent with galaxies undergoing a quasar phase while in the blue cloud before settling on the red sequence with a lower-luminosity X-ray AGN, if they are similar objects at different evolutionary stages.