Two-point correlation function for X-ray selected AGN

The aim of our work Elyiv et al. 2012 was to study the large-scale structure of different types of AGN using the medium-deep XMM-LSS survey. We measured the two-point angular correlation function of ∼5700 and 2500 X-ray point-like sources over the ∼11 sq. deg. XMM-LSS field in the soft (0.5–2 keV) and hard (2–10 keV) bands, respectively. Figs. 4 and 5 show LogN-LogS relations for noth samples. For the conversion from the angular to the spatial correlation function we used the Limber integral equation and the luminosity-dependent density evolution model of the AGN X-ray luminosity function.

We found significant angular correlations with the power-law parameters γ = 1.81”± 0.02”, θ0 = 1.3”±0.2” for the soft, and γ = 2.00”±0.04”, θ0 = 7.3”±1.0 10 h−1 Mpc, irrespective of the flux limit, Fig. 8. Our analysis of AGN subsamples with different hardness ratios shows that the sources with a hard-spectrum are more clustered than soft-spectrum ones.

This result may be a hint that the two main types of AGN populate different environments. Finally, we find that our clustering results correspond to an X-ray selected AGN bias factor of ∼2.5 for the soft band sources (at a median z=1.1) and ∼3.3 for the hard band sources (at a median z=1), which translates into a host dark matter halo mass of ∼1013 h−1 Msol and ∼1013.7 h−1 Msol  for the soft and hard bands, respectively.