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Published December 11, 2007 | Published
Journal Article Open

Spectroscopic follow-up of a cluster candidate at z=1.45


We have obtained deep optical spectroscopic data of the highest-redshift cluster candidate (z ~ 1.4, CVB13) selected by Van Breukelen et al. in a photometric optical/infrared catalogue of the Subaru XMM-Newton Deep Field. The data, which comprise 104 targeted galaxies, were taken with the DEep Imaging Multi-Object Spectrograph on the Keck 2 telescope and yielded 31 secure redshifts in the range 1.25 < z < 1.54 within a 7 x 4-arcmin(^2) field centred on CVB13. Instead of one massive cluster at z = 1.4, we find evidence for three projected structures at z = 1.40, 1.45 and 1.48. The most statistically robust of these structures, at z = 1.454, has six spectroscopically confirmed galaxies. Its total mass is estimated at greater than or similar to 10(^14) M☉ and it may therefore be termed a cluster. There is an X-ray source at the cluster position which is marginally spatially resolved but whose X-ray spectrum is too hard to be thermal cluster emission. Its origin could be the summed X-ray emission from active galaxies in, and projected on to, the cluster. Serendipitously, we have discovered a cluster at z = 1.28 with a mass of greater than or similar to 10(^14) M☉ at the same position on the sky, comprising six spectroscopically confirmed cluster galaxies and at least one additional radio source. The selection of CVB13 for the cluster catalogue was evidently aided by the superposition of other, presumably lower mass, structures, whereas the single cluster at z = 1.28 contained too few galaxies to be isolated by the same algorithm. Given the complicated nature of such structures, caution must be employed when measuring the mass function of putative high-redshift clusters with photometric techniques alone.

Additional Information

© 2007 The Authors. Journal compilation © 2007 RAS. Accepted 2007 August 29. Received 2007 August 28; in original form 2007 July 23. The authors are grateful to Lance Miller for sharing his cluster mass function code. CvB would like to thank Oxford Astrophysics for studentship funding and CvB, DB and CS acknowledge funding from the Science and Technology Facilities Council. 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. The analysis pipeline used to reduce the DEIMOS data was developed at UC Berkeley with support from NSF grant AST-0071048.

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