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

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[1] Koulouridis, E.

[2] 1,2,Plionis, M.

[3] 4,Melnyk, O.

[4] 4,6,Elyiv, A.

[5] Georgantopoulos, I.

[6] Clerc, N.

[7] Surdej, J.

[8] Chiappetti, L.

[9] Pierre, M.

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| 1600年 / EDP Sciences卷 / 567期

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We present a study of the overdensity of X-ray-selected active galactic nuclei (AGN) in 33 galaxy clusters in the XMM-LSS field (The XMM-Newton Large Scale Structure Survey); up to redshift z = 1.05 and further divided into a lower (0.14 ≤ z ≤ 0.35) and a higher redshift (0.43 ≤ z ≤ 1.05) subsample. Previous studies have shown that the presence of X-ray-selected AGN in rich galaxy clusters is suppressed; since their number is significantly lower than what is expected from the high galaxy overdensities in the area. In the current study we have investigated the occurrence of X-ray-selected AGN in low (x; bol> = 2.7 - 1043 erg/s) and moderate (x; bol> = 2.4 × 1044 erg/s) X-ray luminosity galaxy clusters in an attempt to trace back the relation between high-density environments and nuclear activity. Owing to the wide contiguous XMM-LSS survey area; we were able to extend the study to the cluster outskirts. We therefore determined the projected overdensity of X-ray point-like sources around each cluster out to 6r500 radius; within; δr500; 1; annulus; with respect to the field expectations based on the X-ray source log N - log S of the XMM-LSS field. To provide robust statistical results we also conducted a consistent stacking analysis separately for the two z ranges. We investigated whether the observed X-ray overdensities are to be expected thanks to the obvious enhancement of galaxy numbers in the cluster environment by also estimating the corresponding optical galaxy overdensities; and we assessed the possible enhancement or suppression of AGN activity in clusters. We find a positive X-ray projected overdensity in both redshift ranges at the first radial bins; which however has the same amplitude as that of optical galaxies. Therefore; no suppression (or enhancement) of X-ray AGN activity with respect to the field is found; in sharp contrast to previous results based on rich galaxy clusters; implying that the mechanisms responsible for the suppression are not as effective in lower density environments. After a drop to roughly the background level between 2 and 3r500; the X-ray overdensity exhibits a rise at larger radii; significantly greater than the corresponding optical overdensity. The radial distance of this overdensity bump; corresponding to ∼1.5-3 Mpc; depends on the richness of the clusters; as well as on the overall X-ray overdensity profile. Finally; using the redshift information; photometric or spectroscopic; of the optical counterparts; we derive the spatial overdensity profile of the clusters.We find that the agreement between X-ray and optical overdensities in the first radial bins is also suggested in the 3-dimensional analysis. However; we argue that the X-ray overdensity bump at larger radial distance is at least partially a result of flux boosting by gravitational lensing of background quasi-stellar objects; confirming previous results. For high-redshift clusters; the enhancement of X-ray AGN activity in their outskirts appears to be intrinsic. We argue that a spatial analysis is crucial for disentangling irrelevant phenomena affecting the projected analysis; but we are still not able to report statistically significant results on the spatial overdensity of AGN in clusters or their outskirts because we lack the necessary numbers. © ESO 2014;

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