Tracing the Large‐Scale Structure in theChandraDeep Field South

Abstract

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 (Dzd0:02) spikes in the source redshift distribution at z ¼ 0:67 and 0.73, respectively. Their angular distribution spans a region at least $17 0 wide, corresponding to a physical size of 7:3 h À1 70 Mpc at a redshift of z $ 0:7 ( m ¼ 0:3, Ã ¼ 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 ¼ 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 ¼ 0:67 and 0.73 (again with Dz $ 0:02) are the most significant features, containing 24 and 47 galaxies, respectively. While the K20 structure at z ¼ 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 ¼ 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 ¼ 0:73 the fraction of X-ray sources is the same as in the field, in the structure at z ¼ 0:67 it is higher by a factor of $2, suggesting that X-ray activity may be triggered preferentially in the structure at z ¼ 0:67. Given the limited statistics, this result is significant only at the $2 level.

A new (2+1)D cluster finding algorithm based on photometric redshifts: large scale structure in the Chandra deep field south

“…Two main clumps appear about redshifts 0.70, 1.00. A comparison with the distribution of spectroscopic redshifts of the CDFS (see Gilli et al 2003, Fig. 1) clearly indicates the reality of the two clumps, although the two peaks at z = 0.67 and z = 0.73 found by Gilli et al (2003) in the distribution of spectroscopic redshifts are barely resolved in our photometric redshift distribution.…”

“…The first structure, at z = 0.70 is approximately centred on the cD galaxy, whose position, in turn, corresponds to the centre of the extended X-ray source CDFS566 (Giacconi et al 2002). As noted above, we cannot resolve, in redshift space, the walllike structure at z = 0.67 described by Gilli et al (2003) which then contaminates the structure at z = 0.70. In spite of that we find a relatively concentrated structure with full width at half maximum of about 0.12 Mpc.…”

“…The existence of deep X-ray data from the Chandra observatory provided a sample of Active Galactic Nuclei (AGN) in the field (Giacconi et al 2002). The spectroscopic follow up has shown the existence of large-scale structure (Gilli et al 2003). AGN, thanks to their intrinsic luminosity and to strong emission lines, are the ideal tracers of the mass distribution at the maximum distance reachable by optical spectroscopy, under the assumption that their spatial distribution mimics that of normal galaxies.…”

“…The Abell richness of the third structure at z ∼ 1.55 cannot be evaluated, since m 3 + 2 falls below the limiting magnitude m I = 25. The peak in the photometric redshift distribution contains 57 objects spread along a moderate over-density crossing the field from north-west to south-east likely related to the above discussed large-scale structure traced by X-ray selected AGN (Gilli et al 2003). The association of an environmental density with individual galaxies allows both a further assessment of the nature of the detected overdensities and the analysis of the relation between galaxy spectral energy distribution and the environment.…”

A comprehensive study of large-scale structures in the GOODS-SOUTH field up to ${\mathsf z} \sim $ 2.5

“…Our first application to the K20 survey (Cimatti et al 2002) detected two clusters at z ∼ 0.7 and z ∼ 1 ), previously identified through spectroscopy (Gilli et al 2003;Adami et al 2005). We then applied the algorithm to the much larger GOODS-South field, and in Castellano et al (2007), elsewhere C07, we reported our initial results, i.e.…”