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Published May 1, 2009 | Published
Journal Article Open

Evidence of a link between the evolution of clusters and their AGN fraction


We discuss the optical properties, X-ray detections and active galactic nucleus (AGN) populations of four clusters at z ~ 1 in the Subaru–XMM Deep Field (SXDF). The velocity distribution and plausible extended X-ray detections are examined, as well as the number of X-ray point sources and radio sources associated with the clusters. We find that the two clusters that appear virialized and have an extended X-ray detection contain few, if any, AGN, whereas the two pre-virialized clusters have a large AGN population. This constitutes evidence that the AGN fraction in clusters is linked to the clusters' evolutionary stage. The number of X-ray AGN in the pre-virialized clusters is consistent with an overdensity of a factor of ~200; the radio AGN appear to be clustered with a factor of 3 to 6 higher. The median K-band luminosities of L_K= 1.7 ± 0.7 L* for the X-ray sources and L_K= 2.3 ± 0.1 L* for the radio sources support the theory that these AGN are triggered by galaxy interaction and merging events in sub-groups with low internal velocity distributions, which make up the cluster environment in a pre-virialization evolutionary stage.

Additional Information

© 2009 The Authors. Journal compilation © 2009 RAS. Accepted 2009 January 15. Received 2009 January 15; in original form 2008 August 21. The authors would like to thank Doi, Morokuma, Miyazaki, Saito, Yamada, Satoshi, Smail and Croom for sharing their redshifts in the SXDF master list. CVB acknowledges support from STFC in the form of a postdoctoral fellowship. This paper is partly based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministrio da Cincia e Tecnologia (Brazil) and SECYT (Argentina). Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

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