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Published January 1, 2008 | Published
Journal Article Open

Chandra Observations of the cl1604 Supercluster at z = 0.9: Evidence for an Overdensity of Active Galactic Nuclei


We present the results of Chandra observations of the Cl1604 supercluster at z ~ 0.9. The system is the largest structure mapped at redshifts approaching unity, containing at least eight spectroscopically confirmed galaxy clusters and groups. Using two 50 ks ACIS-I pointings we examine both the X-ray point-source population and the diffuse emission from individual clusters in the system. Over an area encompassing the entire supercluster (~0.154 deg2), we find a 2.5σ excess of point sources detected in the hard band (2-10 keV) relative to the number of sources found in blank fields observed by Chandra. No such excess is observed in the soft band (0.5-2 keV). The hard-band source density is 1.47 times greater than that of a blank field, in agreement with the previously reported correlation between overdensity amplitude and cluster redshift. Using a maximum likelihood technique, we have matched 112 of the 161 detected X-ray point sources to optical counterparts and found 15 sources that are associated with the supercluster. All 15 sources have rest-frame luminosities consistent with emission from active galactic nuclei (AGNs). We find that the supercluster AGNs largely avoid the densest regions of the system and are instead distributed on the outskirts of massive clusters or within poorer clusters and groups. We have also detected diffuse emission from two of the eight clusters and groups in the system, clusters Cl1604 + 4304 and Cl1604 + 4314. The systems have bolometric luminosities of 1.43 × 10^44 and 8.20 × 10^43 h70^–2 erg s^–1 and gas temperatures of 3.50+1.82 –1.08 and 1.64+0.65 –0.45 keV, respectively. Using updated velocity dispersions, we compare the properties of these systems with the cluster scaling relations followed by other X-rays and optically selected galaxy clusters at high redshift.

Additional Information

© 2009 The American Astronomical Society. Received 2008 April 11, accepted for publication 2008 August 11. Published 2008 December 1. Print publication: Issue 1 (2009 January 1). We thank Patrick Henry and Harald Ebeling for several useful discussions. This work is supported by the Chandra General Observing Program under award number GO6-7114X. Additional support for this program was provided by NASA through a grant HST-GO-11003 from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. The spectrographic data used 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. Online-only material: color figures, machine-readable tables.

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