Excess AGN activity in the z = 2.30 Protocluster in HS 1700+64

J. A. Digby-North, K. Nandra, E. S. Laird, C. C. Steidel, A. Georgakakis, M. Bogosavljević, D. K. Erb, A. E. Shapley, N. A. Reddy, J. Aird

Research output: Contribution to journalArticlepeer-review


We present the results of spectroscopic, narrow-band and X-ray observations of a z = 2.30 protocluster in the field of the QSO HS 1700+643. Using a sample of BX/MD galaxies, which are selected to be at z ∼ 2.2-2.7 by their rest-frame ultraviolet colours, we find that there are five protocluster AGN which have been identified by characteristic emission-lines in their optical/near-IR spectra; this represents an enhancement over the field significant at >98.5 per cent confidence. Using a ∼200-ks Chandra/ACIS-I observation of this field we detect a total of 161 X-ray point sources to a Poissonian false-probability limit of 4 × 10-6 and identify eight of these with BX/MD galaxies. Two of these are spectroscopically confirmed protocluster members and are also classified as emission-line AGN. When compared to a similarly selected field sample, the analysis indicates this is also evidence for an enhancement of X-ray selected BX/MD AGN over the field, significant at >99 per cent confidence. Deep Lyα narrow-band imaging reveals that a total of 4/123 Lyα emitters (LAEs) are found to be associated with X-ray sources, with two of these confirmed protocluster members and one highly likely member. We do not find a significant enhancement of AGN activity in this LAE sample over that of the field (result is significant at only 87 per cent confidence). The X-ray emitting AGN fractions for the BX/MD and LAE samples are found to be 6.9-4.4+9.2 and 2.9-1.6+2.9 per cent, respectively, for protocluster AGN with L2-10 keV ≥ 4.6 × 1043 erg s-1 at z = 2.30. These findings are similar to results from the z = 3.09 protocluster in the SSA 22 field found by Lehmer et al. (2009), in that both suggest AGN activity is favoured in dense environments at z > 2.

Original languageEnglish (US)
Pages (from-to)846-853
Number of pages8
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
StatePublished - Sep 2010


  • Galaxies: active
  • Galaxies: clusters: general
  • Galaxies: high-redshift
  • Surveys
  • X-rays: general

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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