Environmental properties of X-ray selected AGN


According to the unified scheme of Antonucci (1993), AGN of different types, like Seyfert type I and II, and broad and narrow line QSOs (unobscured and obscured ones), should
inhabit similar environments, since these objects only diff er because of the orientation of their torus with respect to the line-of-sight. However, the standard orientation-based AGN unification scheme does not consider any evolution with redshift of the properties of obscured vs. unobscured AGN. Is there some observational evidence establishing a similar environment of the different types of AGN? Statistical studies lead to contradictory results.

For example, Koulouridis et al. (2006) found in the very local Universe that the fraction of Seyfert 2 galaxies with close neighbors is significantly larger than that of their control sample and of Seyfert 1 galaxies, while their large scale environment does not show any diff erence compared to their control samples (see also Sorrentino et al. 2006 for similar results). At the same time, Strand et al. (2008) have shown that higher luminosity AGN inhabit more overdense environments compared to lower luminosity AGN to 2 Mpc. The authors also found that type II and type I QSOs present similarly overdense environments in the redshift range of 0.3< z <0.6, while the environment of dimmer type I quasars appears to be less overdense than that of type II quasars.

The main aim of our work Melnyk et al. (2013) is to consider the environmental properties of different types of X-ray selected XMM-LSS sources using the local density of optical galaxies based on the CFHTLS1. In our analysis, we use the newest public version of the XMM-LSS multiwavelength catalog (Chiappetti et al. 2013), which contains 6342 X-ray sources  over 11.1 sq. deg.

We found that although X-ray sources may be found in variety of environments, a high fraction (∼ > 55-60%), as verified by comparing with the random expectations, reside in overdense regions. The galaxy overdensities within which X-ray sources reside show a positive recent redshift evolution (at least for the range studied; 0.1< z < 0.85), fig. 10 upper panel. We also found that X-ray selected galaxies, when compared to AGN, inhabit significantly higher galaxy overdensities, although their spatial extent appear to be smaller than that of AGN (fig. 11). We showed that Hard AGN (HR ≥ −0.2, AGN type 2) are located in more overdense regions than soft AGN (HR < −0.2, AGN type 1), which is clearly seen in both redshift ranges, although it appears to be stronger in the higher redshift range (0.55 < z < 0.85), fig. 12. This result generally agrees with the correlation function analysis of the XMM-LSS X-ray point-source catalog (having a median z ∼  1) presented in Elyiv et al. (2012), in which the clustering of the Hard AGN was found to be stronger than that of the soft AGN. Furthermore, the galaxy overdensities (with δ > 1.5) within which soft AGN are embedded appear to evolve more rapidly compared to the corresponding overdensities around hard AGN (fig. 10, lower panel).