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Published March 1, 2013 | Published
Journal Article Open

A Refined Estimate of the Ionizing Emissivity from Galaxies at z ≃ 3: Spectroscopic Follow-up in the SSA22a Field

Abstract

We investigate the contribution of star-forming galaxies to the ionizing background at z ~ 3, building on previous work based on narrowband (NB3640) imaging in the SSA22a field. We use new Keck/LRIS spectra of Lyman break galaxies (LBGs) and narrowband-selected Lyα emitters (LAEs) to measure redshifts for 16 LBGs and 87 LAEs at z > 3.055, such that our NB3640 imaging probes the Lyman-continuum (LyC) region. When we include the existing set of spectroscopically confirmed LBGs, our total sample with z > 3.055 consists of 41 LBGs and 91 LAEs, of which 9 LBGs and 20 LAEs are detected in our NB3640 image. With our combined imaging and spectroscopic data sets, we critically investigate the origin of NB3640 emission for detected LBGs and LAEs. We remove from our samples three LBGs and three LAEs with spectroscopic evidence of contamination of their NB3640 flux by foreground galaxies and statistically model the effects of additional, unidentified foreground contaminants. The resulting contamination and LyC-detection rates, respectively, are 62% ± 13% and 8% ± 3% for our LBG sample, and 47% ± 10% and 12% ± 2% for our LAE sample. The corresponding ratios of non-ionizing UV to LyC flux density, corrected for intergalactic medium (IGM) attenuation, are 18.0^(+34.8)_(–7.4) for LBGs and 3.7^(+2.5)_(–1.1) for LAEs. We use these ratios to estimate the total contribution of star-forming galaxies to the ionizing background and the hydrogen photoionization rate in the IGM, finding values larger than, but consistent with, those measured in the Lyα forest. Finally, the measured UV to LyC flux-density ratios imply model-dependent LyC escape fractions of f^(LyC)_(esc) ~ 5%-7% for our LBG sample and f^(LyC)_(esc) ~ 10%-30% for our fainter LAE sample.

Additional Information

© 2013 American Astronomical Society. Received 2012 October 3; accepted 2012 December 28; published 2013 February 14. 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. We thank Gwen Rudie for providing us with updated measurements of the z ∼ 2–3 IGM opacity, and Gwen Rudie and Milan Bogosavljević for their assistance in the collection of some of the data used in this paper. D.B.N., A.E.S., and K.A.K 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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