The chandra deep protocluster survey: Evidence for an enhancement of AGN activity in the SSA22 protocluster at z = 3.09

B. D. Lehmer, D. M. Alexander, J. E. Geach, Ian Smail, A. Basu-Zych, F. E. Bauer, S. C. Chapman, Y. Matsuda, C. A. Scharf, M. Volonteri, T. Yamada

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81 Citations (Scopus)


We present results from a new ultra-deep ≈400 ks Chandra observation of the SSA22 protocluster at z = 3.09. We have studied the X-ray properties of 234 z ∼ 3 Lyman Break Galaxies (LBGs; protocluster and field) and 158 z = 3.09 Lyα Emitters (LAEs) in SSA22 to measure the influence of the high-density protocluster environment on the accretion activity of supermassive black holes (SMBHs) in these UV-selected star-forming populations. We detect individually X-ray emission from active galactic nuclei (AGNs) in six LBGs and five LAEs; due to small overlap between the LBG and LAE source population, ten of these sources are unique. At least six and potentially eight of these sources are members of the protocluster. These sources have rest-frame 8-32 keV luminosities in the range of L 8-32 keV = (3-50) ×1043 ergs s-1and an average observed-frame 2-8 keV to 0.5-2 keV band ratio (BR) of ≈0.8 (mean effective photon index of Γeff≈ 1.1), suggesting significant absorption columns of N H ≳ 10 22-1024 cm-2. We find that the fraction of LBGs and LAEs in the z = 3.09 protocluster harboring an AGN with L 8-32 keV ≳ 3 × 1043 ergs s-1is 9.5-6.1% +12.7 and 5.1-3.3% +6.8, respectively. These AGN fractions are somewhat larger (by a mean factor of 6.1+10.3 -3.6; significant at the ≈95% confidence level) than z ∼ 3 sources found in lower-density "field" environments. Theoretical models imply that these results may be due to the presence of more actively growing and/or massive SMBHs in LBGs and LAEs within the protocluster compared to the field. Such a result is expected in a scenario where enhanced merger activity in the protocluster drives accelerated galaxy and SMBH growth at z≳ 2-3. Using Spitzer IRAC imaging we found that the fraction of IRAC-detected LBGs is significantly larger in the protocluster than in the field (by a factor of 3.0+2.0 -1.3). From these data, we constrained the median rest-frame H-band luminosity in the protocluster to be ≳ 1.2-1.8 times larger than that for the field. When combined with our X-ray data, this suggests that both galaxies and SMBHs grew more rapidly in protocluster environments.

Original languageEnglish
Pages (from-to)687-695
Number of pages9
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 2009 Jan 20
Externally publishedYes


  • X-rays: general
  • cosmology: observations
  • early universe
  • galaxies: active
  • galaxies: clusters: general
  • surveys

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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