Clustering of X-Ray-Selected AGN
  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.
