Tracing the Large-Scale Structure in the Chandra Deep Field South

We report the discovery of large-scale structures of X-ray sources in the 1 Ms observation of the Chandra Deep Field South. Two main structures appear as narrow (Δz ≲ 0.02) spikes in the source redshift distribution at z = 0.67 and 0.73, respectively. Their angular distribution spans a region at least ~17' wide, corresponding to a physical size of 7.3 h Mpc at a redshift of z ~ 0.7 (Ωm = 0.3, ΩΛ = 0.7). These spikes are populated by 19 sources each, which are mainly identified as active galactic nuclei (AGNs). Two sources in each spike are extended in X-rays, corresponding to galaxy groups/clusters embedded in larger structures. The X-ray source redshift distribution shows other spikes, the most remarkable at z = 1.04, 1.62, and 2.57. This is one of the first evidences for large-scale structure traced by X-ray sources and for spatial clustering of X-ray-selected AGNs. The X-ray data have been complemented with the spectroscopic data from the K20 near-infrared survey (Cimatti et al.), which covers ~1/10 of the X-ray field. In this survey, too, the source redshift distribution shows several spikes. Two narrow structures at z = 0.67 and 0.73 (again with Δz ~ 0.02) are the most significant features, containing 24 and 47 galaxies, respectively. While the K20 structure at z = 0.73 is dominated by a standard galaxy cluster with a significant concentration around a central cD galaxy and morphological segregation, the galaxies at z = 0.67 constitute a loose structure rather uniformly distributed along the K20 field. Moreover, we find a very good correlation (almost one-to-one) between less prominent peaks detected in the redshift distributions of X-ray and K20 sources. In particular, at z < 1.3 we find that five out of the six more significant K20 peaks have a corresponding peak in the X-ray-selected sources and, similarly, all five X-ray peaks below that redshift have a corresponding K20 peak. Since the K20 survey sensitivity drops beyond z ~ 1.3, structures at higher redshift are traced only by the X-ray sources. This correlation suggests that AGNs (from the X-ray data) and (early-type) galaxies (from the K20 survey) are tracing the same underlying structures. We also compared the X-ray and K-band catalogs to search for enhanced X-ray activity in the sources in the two main redshift spikes. While in the structure at z = 0.73 the fraction of X-ray sources is the same as in the field, in the structure at z = 0.67 it is higher by a factor of ~2, suggesting that X-ray activity may be triggered preferentially in the structure at z = 0.67. Given the limited statistics, this result is significant only at the ~2 σ level.