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The Lyman Continuum Escape Fraction of the Cosmic Horseshoe: A Test of Indirect Estimates

Vasei, Kaveh and Siana, Brian and Shapley, Alice E. and Quider, Anna M. and Alavi, Anahita and Rafelski, Marc and Steidel, Charles C. and Pettini, Max and Lewis, Geraint F. (2016) The Lyman Continuum Escape Fraction of the Cosmic Horseshoe: A Test of Indirect Estimates. Astrophysical Journal, 831 (1). Art. No. 38. ISSN 0004-637X.

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High-redshift star-forming galaxies are likely responsible for the reionization of the universe, yet direct detection of their escaping ionizing (Lyman continuum [LyC]) photons has proven to be extremely challenging. In this study, we search for escaping LyC of the Cosmic Horseshoe, a gravitationally lensed, star-forming galaxy at z = 2.38 with a large magnification of ~24. Transmission at wavelengths of low-ionization interstellar absorption lines in the rest-frame ultraviolet suggests a patchy, partially transparent interstellar medium. This makes it an ideal candidate for direct detection of the LyC. We obtained a 10-orbit Hubble near-UV image using the Wide Field Camera 3 (WFC3)/UVIS F275W filter that probes wavelengths just below the Lyman limit at the redshift of the Horseshoe in an attempt to detect escaping LyC radiation. After fully accounting for the uncertainties in the opacity of the intergalactic medium (IGM) and accounting for the charge transfer inefficiency in the WFC3 CCDs, we find a 3 σ upper limit for the relative escape fraction of f_(esc,rel) < 0.08. This value is a factor of five lower than the value (0.4) predicted by the 40% transmission in the low-ion absorption lines. Though possible, it is unlikely that the nondetection is due to a high-opacity line of sight through the IGM < 20% chance). We discuss several possible causes for the discrepancy between the escape fraction and the covering fraction and consider the implications for future attempts at both direct LyC detection and indirect estimates of the escape fraction.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Siana, Brian0000-0002-4935-9511
Shapley, Alice E.0000-0003-3509-4855
Alavi, Anahita0000-0002-8630-6435
Rafelski, Marc0000-0002-9946-4731
Steidel, Charles C.0000-0002-4834-7260
Pettini, Max0000-0002-5139-4359
Additional Information:© 2016. The American Astronomical Society. Received 2016 March 8; revised 2016 July 26; accepted 2016 August 6; published 2016 October 26. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. We also thank the referee for comments that helped clarify specific points. Facilities: HST (WFC3 - , UVIS) - , Keck (ESI).
Funding AgencyGrant Number
NASA Postdoctoral ProgramUNSPECIFIED
Subject Keywords:galaxies: individual (Cosmic Horseshoe) ; galaxies: starburst; gravitational lensing: strong; intergalactic medium; ultraviolet: galaxies
Issue or Number:1
Record Number:CaltechAUTHORS:20161027-140937011
Persistent URL:
Official Citation:Kaveh Vasei et al 2016 ApJ 831 38
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:71558
Deposited By: George Porter
Deposited On:28 Oct 2016 14:26
Last Modified:20 Dec 2019 17:05

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