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Keck Spectroscopy of Gravitationally Lensed z ≃ 4 Galaxies: Improved Constraints on the Escape Fraction of Ionizing Photons

Jones, Tucker A. and Ellis, Richard S. and Schenker, Matthew A. and Stark, Daniel P. (2013) Keck Spectroscopy of Gravitationally Lensed z ≃ 4 Galaxies: Improved Constraints on the Escape Fraction of Ionizing Photons. Astrophysical Journal, 779 (1). Art. No. 52. ISSN 0004-637X.

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The fraction of ionizing photons that escape from young star-forming galaxies is one of the largest uncertainties in determining the role of galaxies in cosmic reionization. Yet traditional techniques for measuring this fraction are inapplicable at the redshifts of interest due to foreground screening by the Lyα forest. In an earlier study, we demonstrated a reduction in the equivalent width of low-ionization absorption lines in composite spectra of Lyman break galaxies at z ≃ 4 compared to similar measures at z ≃ 3. This might imply a lower covering fraction of neutral gas and hence an increase with redshift in the escape fraction of ionizing photons. However, our spectral resolution was inadequate to differentiate between several alternative explanations, including changes with redshift in the outflow kinematics. Here we present higher quality spectra of three gravitationally lensed Lyman break galaxies at z ≃ 4 with a spectral resolution sufficient to break this degeneracy of interpretation. We present a method for deriving the covering fraction of low-ionization gas as a function of outflow velocity and compare the results with similar quality data taken for galaxies at lower redshift. We find an interesting but tentative trend of lower covering fractions of low-ionization gas for galaxies with strong Lyα emission. In combination with the demographic trends of Lyα emission with redshift from our earlier work, our results provide new evidence for a reduction in the average H I covering fraction, and hence an increase in the escape fraction of ionizing radiation from Lyman break galaxies, with redshift.

Item Type:Article
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URLURL TypeDescription paper
Jones, Tucker A.0000-0001-5860-3419
Ellis, Richard S.0000-0001-7782-7071
Additional Information:© 2013 American Astronomical Society. Received 2013 April 25; accepted 2013 September 20; published 2013 November 25. We thank Max Pettini for providing spectra of the z = 2–3 galaxies used as a comparison sample. We thank the anonymous referee for a detailed report that significantly improved the content and clarity of this paper. T.A.J. acknowledges support from the Southern California Center for Galaxy Evolution through a CGE Fellowship. D.P.S. acknowledges support from NASA through Hubble Fellowship grant HST-HF-51299.01 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract NAS5-265555. The analysis pipeline used to reduce the DEIMOS data was developed at UC Berkeley with support from NSF grant AST-0071048. This work relies 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 the National Aeronautics and Space Administration, and was made possible by the generous financial support of the W. M. Keck Foundation. We wish to recognize the significant cultural role that the summit of Mauna Kea has within the indigenous Hawaiian community; we are most fortunate to have the opportunity to conduct observations from this mountain.
Funding AgencyGrant Number
Southern California Center for Galaxy Evolution CGE FellowshipUNSPECIFIED
NASA Hubble Fellowship GrantHST-HF-51299.01
W. M. Keck FoundationUNSPECIFIED
Subject Keywords: dark ages, reionization, first stars; galaxies: evolution; galaxies: formation; galaxies: ISM
Issue or Number:1
Record Number:CaltechAUTHORS:20140117-140125452
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Official Citation:Keck Spectroscopy of Gravitationally Lensed z 4 Galaxies: Improved Constraints on the Escape Fraction of Ionizing Photons Tucker A. Jones et al. 2013 ApJ 779 52
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:43437
Deposited By: Ruth Sustaita
Deposited On:17 Jan 2014 22:25
Last Modified:09 Mar 2020 13:18

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