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Published July 20, 2009 | Published
Journal Article Open

Serendipitous Discovery of an Overdensity of Lyα Emitters at z ~ 4.8 in the CL1604 Supercluster Field


We present results of a spectroscopic search for Lyα emitters (LAEs) in the Cl1604 supercluster field using the extensive spectroscopic Keck/DEep Imaging Multi-Object Spectrograph database taken as part of the Observations of Redshift Evolution in Large-Scale Environments survey. A total of 12 slitmasks were observed and inspected in the Cl1604 field, spanning a survey volume of 1.365 × 10^4 comoving Mpc^3. We find a total of 17 high-redshift (4.39 ≤ z ≤ 5.67) LAE candidates down to a limiting flux of 1.9 × 10^(–18) erg s^(–1) cm^(–2) (L(Lyα) = 4.6 × 10^(41) erg s^(–1) or ~0.1 L _* at z ~ 5), 13 of which we classify as high quality. The resulting LAE number density is nearly double that of LAEs found in the Subaru deep field at z ~ 4.9 and nearly an order of magnitude higher than in other surveys of LAEs at similar redshifts, an excess that is essentially independent of LAE luminosity. We also report on the discovery of two possible LAE group structures at z ~ 4.4 and z ~ 4.8 and investigate the effects of cosmic variance of LAEs on our results. Fitting a simple truncated single Gaussian model to a composite spectrum of the 13 high-quality LAE candidates, we find a best-fit stellar velocity dispersion of 136 km s^(–1). Additionally, we see modest evidence of a second peak in the composite spectrum, possibly caused by galactic outflows, offset from the main velocity centroid of the LAE population by ~440 km s^(–1) as well as evidence for a nontrivial Lyα escape fraction. We find an average star formation rate density (SFRD) of ~5 × 10^(–3) M_☉ yr^(–1) Mpc^(–3) with moderate evidence for negative evolution in the SFRD from z ~ 4.6 to z ~ 5.7. By simulating the statistical flux loss due to our observational setup, we measure a best-fit luminosity function characterized by Φ_* L_* = 2.2^(+3.9)_(–1.3) × 10^(39) erg s^(–1) Mpc^(–3) for α = –1.6, generally consistent with measurements from other surveys at similar epochs. Finally, we investigate any possible effects from weak or strong gravitational lensing induced by the foreground supercluster, finding that our LAE candidates are minimally affected by lensing processes.

Additional Information

© 2009. The American Astronomical Society. Received 2008 December 17; accepted 2009 April 24; published 2009 June 30. We thank Evan Kirby and Jeffrey Newman for useful discussions on co-addition techniques and for kindly providing their codes and Nick Konidaris for helpful advice and guidance with DEIMOS flux calibration. We thank Raja Guhathakurta for the idea. D.J.L. and B.C.L. thank Matt Auger for useful discussions and criticisms. We also thank the anonymous referee for useful comments and suggestions. B.C.L. wishes to thank Eric Opland and Carl Olding for making this work possible. B.C.L. also wishes to thank Margaret Thompson for careful reading of the paper and for the many grammar corrections: appear up here! This material is based upon work supported by the National Aeronautics and Space Administration under Award NNG05GC34ZG for the Long Term Space Astrophysics Program. The research of M.S. is financially supported by the Natural Science and Engineering Research Council of Canada and the Canadian Space Agency. Based in part on data collected at Subaru Telescope and obtained from the SMOKA, which is operated by the Astronomy Data Center, National Astronomical Observatory of Japan. The spectrographic 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. 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. We wish to thank the indigenous Hawaiian community for allowing us to be guests on their sacred mountain; we are most fortunate to be able to conduct observations from this site.

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