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Published July 20, 2011 | Published
Journal Article Open

Narrowband Imaging of Escaping Lyman-continuum Emission in the SSA22 Field

Abstract

We present the results of an ultradeep, narrowband imaging survey for Lyman-continuum (LyC) emission at z ~ 3 in the SSA22a field. We employ a custom narrowband filter centered at λ = 3640 Å (NB3640), which probes the LyC region for galaxies at z ≥ 3.06. We also analyze new and archival NB4980 imaging tuned to the wavelength of the Lyα emission line at z = 3.09, and archival broadband B, V, and R images of the non-ionizing UV continuum. Our NB3640 images contain 26 z ≥ 3.06 Lyman break galaxies (LBGs) as well as a set of 130 Lyα emitters (LAEs), identified by their excess NB4980 flux relative to the BV continuum. Six LBGs and 28 LAEs are detected in the NB3640 image. LBGs appear to span a range of NB3640-R colors, while LAEs appear bimodal in their NB3640-R properties. We estimate average UV-to-LyC flux density ratios, corrected for foreground contamination and intergalactic medium absorption, finding (F_(UV)/F_(LyC)^(LBG)_(corr) = 11.3^(+10.3)_(–5.4), which implies an LBG LyC escape fraction f^(LyC)_(esc) ~ 0.1, and (F_(UV)/F_(LyC)^(LAE)_(corr) = 2.2^(+0.9)_(–0.6). The strikingly blue LAE flux density ratios defy interpretation in terms of standard stellar population models. Assuming (F _(UV)/F_(LyC)^(LBG)_(corr) applies down to L = 0.1L^*, we estimate a galaxy contribution to the intergalactic hydrogen ionization rate that is consistent with independent estimates based on the Lyα forest opacity at z sime 3. If we assume that (F_(UV)/F_(LyC)^(LAE)_(corr) holds at the faintest luminosities, the galaxy contribution significantly exceeds that inferred from the Lyα forest. We interpret our results in terms of a model where LyC photons escape over only ~10%-20% of solid angle. When advantageously oriented, a galaxy will exhibit a low UV-to-LyC ratio, an effect enhanced for more compact galaxies. This model, however, does not adequately explain the extremely blue NB3640-R colors measured for some LAEs in our sample. Further follow-up study of these faint LAEs is crucial, given the potentially important contribution similar objects make to the process of reionization.

Additional Information

© 2011 The American Astronomical Society. Received 2011 January 31; accepted 2011 April 26; published 2011 June 29. Based, in part, on data 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 NASA, and was made possible by the generous financial support of the W. M. Keck Foundation. Based, in part, on data collected at the Subaru Telescope and obtained from the SMOKA archive, which is operated by the Astronomy Data Center, National Astronomical Observatory of Japan. We thank Andrew Blain for permitting our use of the NB3640 filter, Kevin Hainline for his helpful discussions regarding our photometric simulations, and the anonymous referee for constructive suggestions. D.B.N. and A.E.S. acknowledge support from the David and Lucile Packard Foundation. C.C.S. acknowledges additional support from the John D. and Catherine T. MacArthur Foundation, the Peter and Patricia Gruber Foundation, and NSF grants AST-0606912 and AST-0908805. We wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, most of the observations presented herein would not have been possible.

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August 22, 2023
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