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

Abstract

Context. 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 the XMM-Newton to a flux of 3 × 10 −15 erg s −1 cm −2 in the [0.5−2] keV band. The vast majority of these sources have optical (CFHTLS), infrared (SWIRE IRAC and MIPS), near-infrared (UKIDSS), and/or ultraviolet (GALEX) counterparts. Aims. We wish to investigate the environmental properties of the different types of the XMM-LSS X-ray sources by defining their environment using the i -band CFHTLS W1 catalog of optical galaxies to a magnitude limit of 23.5 mag. Methods. We have classified 4435 X-ray selected sources on the basis of their spectra, SEDs, and X-ray luminosity, and estimated their photometric redshifts, which have a 4−11 band photometry with an accuracy of σ z/(1+zsp) = 0.076 with 22.6% outliers for i < 26 mag. We estimated the local overdensities of 777 X-ray sources that have spectro-z or photo-z calculated by using more than seven bands (accuracy of σ z/(1+zsp) = 0.061 with 13.8% outliers) within the volume-limited region defined by 0.1 ≤ z ≤ 0.85 and −23.5 < M i < −20. Results. 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; z < ∼ 0.85). We also find 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. Hard AGN (HR ≥ −0.2) are located in more overdense regions than soft AGN (HR < −0.2), which is 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 δ > ∼ 1.5) within which soft AGN are embedded appear to evolve more rapidly compared to the corresponding overdensities around hard AGN.

X-ray AGN in the XMM-LSS galaxy clusters: no evidence of AGN suppression

“…The bulk of our sources, with unavalaible spectroscopic redshifts, are from the above subsample, or they have photometric redshifts calculated with at least seven bands. The latter have PDZ < 100, meaning larger uncertainties, although the spectro-z to photo-z relation is similat to that of the PDZ = 100 sources (see Melnyk et al 2013). The LePhare code also indicates secondary solutions, but in this case the sources were rejected.…”

“…Spectroscopy, however, is only available for ∼24% of our X-ray point-like sources (Table 1), and thus we investigate the rest of the objects based on photometric redshifts and visual inspection. Calculation of the photometric redshifts is described shortly in this section and in more detail in Melnyk et al (2013). For each X-ray source, we take only one best rank optical CFHTLS counterpart into account, based on its distance from the X-ray source and its relative brightness (rank = 0 for a single very reliable counterpart or 1 for a less reliable but preferred counterpart, see Chiappetti et al 2013, for details).…”

“…We therefore only considered 4555 pointlike X-ray sources (∼72% completeness), 4450 of which have spectroscopic or photometric redshifts with z > 0 (non-stars). The list of redshifts for all XMM-LSS sources can be found in Table 2 of Melnyk et al (2013). For the photo-z determination, the LePhare 4 public code (Arnouts et al 1999;Ilbert et al 2006) was used.…”

“…Double-peak solutions are produced when only a few photometry bands are available, and that happens mostly in the ABC supplementary fields. Any X-ray counterpart to be considered as a cluster member should have its spectroscopic redshift within ±2000 km s −1 of the cluster redshift z cl , or photometric redshift z ph within σ(1 + z cl ), where σ = 0.065 to 0.076 depending on the available photometry bands that were used for the calculation and the redshift probability distribution (Melnyk et al 2013).…”

The XXL Survey

“…For example, the Hard Bright Serendipitous Survey (HBSS) covers 25 deg 2 reaching a flux limit of F 4.5−10 keV = 7 × 10 −14 erg s −1 cm 2 (Della Ceca et al 2004). The XMM-LSS survey ), a survey with XMMNewton covering contiguously 11 deg 2 , is a good compromise between sky area and depth which has provided several results on the environmental properties of AGN (Elyiv et al 2012;Melnyk et al 2013;Koulouridis et al 2014, showing in particular different behaviors between objects having soft and hard X-ray spectra.…”