Tracing the large-scale structure in the Chandra Deep Field South

R. Gilli, A. Cimatti, E. Daddi, G. Hasinger, P. Rosati, G. Szokoly, P. Tozzi, J. Bergeron, S. Borgani, R. Giacconi, L. Kewley, V. Mainieri, M. Mignoli, M. Nonino, C. Norman, J. Wang, G. Zamorani, W. Zheng, A. Zirm

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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 70-1 Mpc at a redshift of z ∼ 0.7 (ω m = 0.3, ωm = 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 ε level.

Original languageEnglish
Pages (from-to)721-727
Number of pages7
JournalAstrophysical Journal
Issue number2 I
Publication statusPublished - Aug 1 2003


  • Galaxies: clusters: general
  • Galaxies: high-redshift
  • Large-scale structure of universe
  • X-rays: general

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Gilli, R., Cimatti, A., Daddi, E., Hasinger, G., Rosati, P., Szokoly, G., Tozzi, P., Bergeron, J., Borgani, S., Giacconi, R., Kewley, L., Mainieri, V., Mignoli, M., Nonino, M., Norman, C., Wang, J., Zamorani, G., Zheng, W., & Zirm, A. (2003). Tracing the large-scale structure in the Chandra Deep Field South. Astrophysical Journal, 592(2 I), 721-727.