Left: Optical image of X-ray sources in the AEGIS survey, which according to our analysis (based on optical spectroscopy; look below for details) lie in a high density region. An overdensity of optical galaxies in the vicinity of the X-ray sources is apparent to the eye. Right: Overdensity estimator plotted against optical absolute magnitude for X-ray sources and optically selected galaxies in the DEEP2 survey. Each point is the mean within different M_B bins. The uncertainties correspond to the standard error. This figure shows that X-ray selected AGN avoid poor environments and lie, on average, in high density regions

A wide range of galaxy properties such star-formation, colour and morphology strongly depend on environment, e.g. cluster, group or field. This indicates that galaxy formation and evolution is affected and moderated by the large scale environment in which the system resides. In the past few years there has been increasing evidence that black accretion and host galaxy formation/evolution are strongly interconnected. Therefore putting the above two pieces of information together, one would expect a dependence of AGN activity on local density. Such a link however, remains controversial.

One of the problems in addressing this issue is the selection of AGN samples. Different selection criteria can bias the resulting sample against certain types of objects (e.g. obscured or low-luminosity AGN), introducing uncertainties in any statistical analysis. X-ray surveys, especially at hard energies (e.g. 2-8keV) provide the least biased method for identifying AGN.

Using the XMM-Newton/2dF survey we have estimated the angular correlation function of X-ray sources selected in both the hard (2-8keV) and soft (0.5-2keV) spectral bands. We show that this is consistent with correlation lengths in the range 5-10Mpc, depending somewhat on the assumptions of the model used to interpret the data (Basilakos et al. 2004; 2005). Such a large clustering length suggests that AGN reside in rich environments, similar (if not denser) to those of early type galaxies (e.g. groups of clusters).

In addition to the studies above we are also directly measuring the local density in the vicinity of X-ray sources using data from the AEGIS survey. This project combines deep Chandra (200ks) data over a 0.5deg² field of view with extensive optical spectroscopy (obtained as part of the DEEP2 survey) in the Extended Groth Strip region. The redshifts of optical galaxies in the vicinity of an X-ray source are used to estimate the local density relative to the mean, using a friends-of-friends type algorithm (Cooper et al. 2005; 2006). This method not only provides a measure of the local overdensity but also allows one to explore possible dependence of AGN properties (e.g. luminosity, obscuration) on the large scale environment. Initial results suggest that X-ray selected AGN avoid, on average, underdense regions (see figure). This suggest that the AGN activity is linked to local density, e.g. infall of galaxies into larger structures, condensation of cold gas on group or cluster galaxies via cooling flows (e.g. Croton et al. 2005). Future analysis of the AEGIS data will allow us to explore these scenarios.