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Published December 2009 | Published
Journal Article Open

The Host Galaxies of Swift Dark Gamma-ray Bursts: Observational Constraints on Highly Obscured and Very High Redshift GRBs

Abstract

In this work, we present the first results of our imaging campaign at Keck Observatory to identify the host galaxies of "dark" gamma-ray bursts (GRBs), events with no detected optical afterglow or with detected optical flux significantly fainter than expected from the observed X-ray afterglow. We find that out of a uniform sample of 29 Swift bursts rapidly observed by the Palomar 60 inch telescope through 2008 March (14 of which we classify as dark), all events have either a detected optical afterglow, a probable optical host-galaxy detection, or both. Our results constrain the fraction of Swift GRBs coming from very high redshift (z>7), such as the recent GRB 090423, to between 0.2% and 7% at 80% confidence. In contrast, a significant fraction of the sample requires large extinction columns (host-frame A_V ≳ 1 mag, with several events showing A_V > 2-6 mag), identifying dust extinction as the dominant cause of the dark GRB phenomenon. We infer that a significant fraction of GRBs (and, by association, of high-mass star formation) occurs in highly obscured regions. However, the host galaxies of dark GRBs seem to have normal optical colors, suggesting that the source of obscuring dust is local to the vicinity of the GRB progenitor or highly unevenly distributed within the host galaxy.

Additional Information

© 2009 American Astronomical Society. Print publication: Issue 6 (2009 December); received 2009 April 30; accepted for publication 2009 September 19; published 2009 October 30. J.S.B.'s group is partially supported by the Las Cumbres Observatory Global Telescope Network and NASA/Swift Guest Investigator grant NNG05GF55G. S.B.C. acknowledges generous support from Gary and Cynthia Bengier, the Richard and Rhoda Goldman fund, and National Science Foundation (NSF) grant AST0607485. Support for M.B. was provided by the W. M. Keck Foundation. S.L. was supported by FONDECYT grant N◦1060823. The W.M. Keck Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration (NASA). The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Gemini Observatory 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 Ministerio de Ciencia, Tecnologa e Innovacin Productiva (Argentina). PAIRITEL is operated by the Smithsonian Astrophysical Observatory (SAO) and was made possible by a grant from the Harvard University Milton Fund, a camera loan from the University of Virginia, and continued support of the SAO and UC Berkeley. The PAIRITEL project are further supported by NASA/Swift Guest Investigator grant NNG06GH50G and NNX08AN84G. This research has made use of the NASA/ IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This work additionally made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. We also acknowledge the hard work and efforts of the creators of other essential Web sites, in particular astrometry.net and GRBlog, which greatly assisted in this work. We thank D. A. Kann and our anonymous referee for helpful comments and suggestions, and also thank Derek Fox as well as Pall Jakobsson and collaborators for sharing additional data on bursts within our sample. Finally, we acknowledge the hard work and dedication of the Swift team,whose successfulmission has made this study possible. Facilities: Keck:I (LRIS), Gemini:South (NIRI), PAIRITEL (), PO:1.5m ()

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Created:
August 22, 2023
Modified:
October 19, 2023