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Enhancement of AGN in a protocluster at z=1.6

Krishnan, Charutha; Hatch, Nina A.; Almaini, Omar; Kocevski, Dale; Cooke, Elizabeth A.; Hartley, William G.; Hasinger, Guenther; Maltby, David T.; Muldrew, Stuart I.; Simpson, Chris


Charutha Krishnan

Nina A. Hatch

Professor of Astrophysics

Dale Kocevski

Elizabeth A. Cooke

William G. Hartley

Guenther Hasinger

David T. Maltby

Stuart I. Muldrew

Chris Simpson


We investigate the prevalence of AGN in the high-redshift protocluster Cl 0218.3–0510 at z = 1.62. Using imaging from the Chandra X-ray Telescope, we find a large overdensity of AGN in the protocluster; a factor of 23±9 times the field density of AGN. Only half of this AGN overdensity is due to the overdensity of massive galaxies in the protocluster (a factor of 11±2), as we find that 17+6−5% of massive galaxies (M∗ > 1010M⊙) in the protocluster host an X-ray luminous AGN, compared to 8±1% in the field. This corresponds to an enhancement of AGN activity in massive protocluster galaxies by a factor of 2.1±0.7 at 1.6s significance. We also find that the AGN overdensity is centrally concentrated, located within 3 arcmin and most pronounced within 1 arcmin of the centre of the protocluster. Our results confirm that there is a reversal in the local anti-correlation between galaxy density and AGN activity, so there is an enhancement of AGN in high-redshift protoclusters. We compare the properties of AGN in the protocluster to the field and find no significant differences in the distributions of their stellar mass, X-ray luminosity, or hardness ratio. We therefore suggest that triggering mechanisms are similar in both environments, and that the mechanisms simply occur more frequently in denser environments.


Krishnan, C., Hatch, N. A., Almaini, O., Kocevski, D., Cooke, E. A., Hartley, W. G., …Simpson, C. (2017). Enhancement of AGN in a protocluster at z=1.6. Monthly Notices of the Royal Astronomical Society, 470(2), 2170-2178.

Journal Article Type Article
Acceptance Date May 24, 2017
Publication Date May 29, 2017
Deposit Date Jun 2, 2017
Publicly Available Date Jun 2, 2017
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Oxford University Press
Peer Reviewed Peer Reviewed
Volume 470
Issue 2
Pages 2170-2178
Keywords galaxies: active – galaxies: clusters: individual: Cl 0218.3−0510
Public URL
Publisher URL
Additional Information This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2017 The authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.


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