Environmental effects on different types of AGNs in Chandra Deep Field South

[[abstract]]We investigate the relation between cosmic structures and active galactic nuclei (AGNs) observed in Chandra Deep Field South (CDFS) to study how the evolution of galaxies was affected by environments (eg, galaxy clusters or cluster mergers). A total of 889 X-ray sources, compiled from CDFS and the Extended Chandra Deep Field South (E-CDFS) surveys, are analyzed in the study. An optical catalog of galaxies with multi-colour photometry in the same field is extracted from COMBO-17, which includes about 11,500 (MR≦-19.19) objects with photometric redshifts between 0.2 and 1.2. Using the cross-correlation functions ω(θ) of X-ray AGN and galaxies, 2D galaxies density distribution, and Friends-of-Friends (FoF) algorithm analyses, we note that Type 1 AGNs (with hardness ratio HR<-0.2) are spatially cross-correlated with the center of high number density of galaxies region (HNDoGR), and the Type 1 AGN to galaxy ratio in these regions tends to be higher than average. Moreover, our analyses also reveal a correlation between Type 2 AGNs (HR>-0.2) and galaxies structure; further analyses indicate the number of Type 2 AGNs is much larger than Type 1 AGNs on the outskirts of HNDoGR (~ 0.5 Mpc), and the Type 2 AGN to galaxy ratio around the HNDoGR (~ 0.5 Mpc ) tends to be higher than average. These results may infer that the large number of Type 2 AGNs appearing at low redshift could be associated with HNDoGRs, which are undetected large scale structures (LSS) composed of faint galaxies in the low-redshift universe, the interaction of galaxies in the outer part of the faint LSS can provide a mechanism to boost the formation of Type 2 AGNs. On the other hand, our analyses do not show obvious evidence that the evolution of AGNs is related to larger scale environment such as cluster mergers. With our results demonstrating the different correlations between Type 1/2 AGNs and HNDoGR, deeper observations in the future can offer more data to analyze the properties of faint LSS with AGNs embedded, and to provide new clues to the origins of the different types of AGNs.