Journal Article

Determining the escape fraction of ionizing photons during reionization with the GRB-derived star formation rate

J. S. B. Wyithe, A. M. Hopkins, M. D. Kistler, H. Yüksel and J. F. Beacom

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 401, issue 4, pages 2561-2571
Published in print February 2010 | ISSN: 0035-8711
Published online January 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2009.15834.x
Determining the escape fraction of ionizing photons during reionization with the GRB-derived star formation rate

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The fraction of ionizing photons that escape their host galaxies and so are able to ionize hydrogen in the inter-galactic medium (IGM) is a critical parameter in the analyses of the reionization era and early galaxy formation. Studies of the reionization history normally suffer from a degeneracy between the unknown values for the efficiency with which high-redshift galaxies turn mass into stars and the escape fraction of ionizing photons. Recent gamma-ray burst (GRB) measurements of the star formation rate density during reionization provide the first opportunity to break this degeneracy. We confront a semi-analytic model for reionization with the GRB-derived star formation rate, as well as observations of the Lyα forest and the CMB. Assuming that UV photons produced in star-forming galaxies dominate the reionization process, we show that the escape fraction of ionizing photons from high-redshift galaxies is ∼5 per cent [log fesc=−1.35 ± 0.15 (68 per cent)] for our fiducial model. This relatively small value of escape fraction follows from the slow evolution in the star formation rate density at z≳ 4 implied by the number counts of high-redshift GRBs. Our derived value is reasonably stable against uncertainties in the modelling, including the implementation of radiative feedback, the possibility of an evolving escape fraction and the unknown shape of the IMF, which in sum contribute ∼0.2 dex of additional systematic uncertainty on the value of escape fraction.

Keywords: galaxies: high-redshift; intergalactic medium; diffuse radiation; cosmology: theory; large scale structure of Universe

Journal Article.  9696 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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