Classification and environmental properties of X-ray selected point-like sources in the XMM-LSS field

TL;DR

This study classifies X-ray sources in the XMM-LSS field and analyzes their environmental properties, revealing that most sources are in overdense regions and that environment varies with source type and redshift.

Contribution

It provides a detailed classification of X-ray sources and investigates their environmental dependence, including the evolution of galaxy overdensities around different source types.

Findings

Majority of X-ray sources are in overdense regions.

X-ray sources show redshift evolution in their environments.

Hard AGN are found in more overdense regions than Soft AGN.

Abstract

The XMM-Large Scale Structure survey, covering an area of 11.1 sq. deg., contains more than 6000 X-ray point-like sources detected with XMM-Newton down to a flux of 3x10^-15 erg s^-1 cm^-2 in the [0.5-2] keV band, the vast majority of which have optical (CFHTLS), infrared (SWIRE), near-infrared (UKIDSS) and/or ultraviolet (GALEX) counterparts. We wish to investigate the environmental properties of the different types of the XMM-LSS X-ray sources, defining their environment using the i-band CFHTLS W1 catalog of optical galaxies down to a magnitude limit of 23.5 mag. We have classified 4435 X-ray selected sources on the basis of their spectra, SEDs and X-ray luminosity and estimated their photometric redshifts, having 4-11 band photometry, with an accuracy sigma=0.076 and 22.6% outliers for i<26 mag. We estimated the local overdensities of 777 X-ray sources which have spectro-z or photo-z calculated using more than 7 bands (accuracy sigma=0.061 with 13.8% outliers) within the volume-limited region defined by 0.1<z<0.85 and -23.5<Mi<-20. Although X-ray sources may be found in variety of environments, a large 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. We also find that X-ray selected galaxies, with respect to AGN, inhabit significantly higher galaxy overdensities, although their spatial extent appear to be smaller than that of AGN. Hard AGN (HR>=-0.2) are located in more overdense regions with respect to the Soft AGN (HR<-0.2), a fact clearly seen in both redshift ranges, although it appears to be stronger in the higher redshift range (0.55<z<0.85). Furthermore, the galaxy overdensities (with d>1.5) within which Soft AGN are embedded appear to evolve more rapidly with respect to the corresponding overdensities around Hard AGN.