Clustering of X-Ray-Selected AGN

TL;DR

This paper reviews decades of research on the clustering of X-ray-selected AGN, revealing their distribution's link to dark matter halos and discussing various AGN triggering mechanisms.

Contribution

It provides a comprehensive review of observational and theoretical advances in X-ray AGN clustering from 1988 to 2011, highlighting the evolution of understanding in the field.

Findings

X-ray AGN clustering reflects dark matter distribution.

AGN are hosted by dark matter halos similar to galaxy groups.

Multiple mechanisms, including galaxy mergers and disk instabilities, may trigger AGN activity.

Abstract

The study of the angular and spatial structure of the X-ray sky has been under investigation since the times of the Einstein X-ray Observatory. This topic has fascinated more than two generations of scientists and slowly unveiled an unexpected scenario regarding the consequences of the angular and spatial distribution of X-ray sources. It was first established from the clustering of sources making the CXB that the source spatial distribution resembles that of optical QSO. It then it became evident that the distribution of X-ray AGN in the Universe was strongly reflecting that of Dark Matter. In particular one of the key result is that X-ray AGN are hosted by Dark Matter Halos of mass similar to that of galaxy groups. This result, together with model predictions, has lead to the hypothesis that galaxy mergers may constitute the main AGN triggering mechanism. However detailed analysis of observational data, acquired with modern telescopes, and the use of the new Halo Occupation formalism has revealed that the triggering of an AGN could also be attributed to phenomena like tidal disruption or disk instability, and to galaxy evolution. This paper reviews results from 1988 to 2011 in the field of X-ray selected AGN clustering.