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The Connection Between Reddening, Gas Covering Fraction, and the Escape of Ionizing Radiation at High Redshift

Reddy, Naveen A. and Steidel, Charles C. and Pettini, Max and Bogosavljević, Milan and Shapley, Alice E. (2016) The Connection Between Reddening, Gas Covering Fraction, and the Escape of Ionizing Radiation at High Redshift. Astrophysical Journal, 828 (2). Art. No. 108. ISSN 0004-637X. doi:10.3847/0004-637X/828/2/108.

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Using a large sample of spectroscopically confirmed z ~ 3 galaxies, we establish an empirical relationship between reddening (E(B-V), neutral gas covering fraction (f_(cov)HI), and the escape of ionizing (Lyman continuum, LyC) photons. Our sample includes 933 galaxies at z ~ 3,121 of which have deep spectroscopic observations (≳7 hr) at 850 ≾ λ_(rest) ≾ 1300 Å with the Low Resolution Imaging Spectrograph on Keck. The high covering fraction of outflowing optically thick HI indicated by the composite pectra of these galaxies implies that photoelectric absorption, rather than dust attenuation, dominates the depletion of LyC photons. By modeling the composite spectra as the combination of an unattenuated stellar spectrum including nebular continuum emission with one that is absorbed by HI and reddened by a line-of-sight extinction, we derive an empirical relationship between E(B-V) and f_(cov) HI. Galaxies with redder UV continua have larger covering fractions of HI characterized by higher line-of-sight extinctions. We develop a model which connects the ionizing escape fraction with E(B-V), and which may be used to estimate the ionizing escape fraction for an ensemble of galaxies. Alternatively, direct measurements of the escape fraction for our sample allow us to constrain the intrinsic LyC-to-UV flux density ratio to be 〈900 Å)/S(1500 Å〉_(int) ≳ 0.20, a value that favors stellar population models that include weaker stellar winds, a flatter initial mass function, and/or binary evolution. Last, we demonstrate how the framework discussed here may be used to assess the pathways by which ionizing radiation escapes from high-redshift galaxies.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Reddy, Naveen A.0000-0001-9687-4973
Steidel, Charles C.0000-0002-4834-7260
Pettini, Max0000-0002-5139-4359
Shapley, Alice E.0000-0003-3509-4855
Additional Information:© 2016 American Astronomical Society. Received 2016 May 10; revised 2016 June 8; accepted 2016 June 8; published 2016 September 8. N.A.R. is supported by an Alfred P. Sloan Research Fellowship, and acknowledges the visitors program at the Institute of Astronomy in Cambridge, UK, where part of this research was conducted. C.C.S. acknowledges NSF grants AST-0908805 and AST-1313472. MB acknowledges support of the Serbian MESTD through grant ON176021. NAR acknowledges Gwen Rudie for useful discussions. We are grateful to the anonymous referee whose comments led to significant improvements in the clarity, content, and presentation of the analysis and results. 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, the observations presented herein would not have been possible.
Funding AgencyGrant Number
Alfred P. Sloan FoundationUNSPECIFIED
Ministry of Education, Science and Technological Development (Serbia)ON176021
Subject Keywords:dark ages, reionization, first stars; dust, extinction; galaxies: evolution; galaxies: formation; galaxies: high-redshift; galaxies: ISM
Issue or Number:2
Record Number:CaltechAUTHORS:20160912-091542652
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Official Citation:Naveen A. Reddy et al 2016 ApJ 828 108
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:70273
Deposited By: Ruth Sustaita
Deposited On:12 Sep 2016 19:29
Last Modified:11 Nov 2021 04:26

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