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The Difficulty Getting High Escape Fractions of Ionizing Photons from High-redshift Galaxies: a View from the FIRE Cosmological Simulations

Ma, Xiangcheng and Kasen, Daniel and Hopkins, Philip F. and Faucher-Giguère, Claude-André and Quataert, Eliot and Kereš, Dušan and Murray, Norman (2015) The Difficulty Getting High Escape Fractions of Ionizing Photons from High-redshift Galaxies: a View from the FIRE Cosmological Simulations. Monthly Notices of the Royal Astronomical Society, 453 (1). pp. 960-975. ISSN 0035-8711.

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We present a series of high-resolution (20–2000 M⊙, 0.1–4 pc) cosmological zoom-in simulations at z ≳ 6 from the Feedback In Realistic Environment (FIRE) project. These simulations cover halo masses 10^9–10^(11) M⊙ and rest-frame ultraviolet magnitude M_(UV) = −9 to −19. These simulations include explicit models of the multi-phase ISM, star formation, and stellar feedback, which produce reasonable galaxy properties at z = 0–6. We post-process the snapshots with a radiative transfer code to evaluate the escape fraction (f_(esc)) of hydrogen ionizing photons. We find that the instantaneous f_(esc) has large time variability (0.01–20 per cent), while the time-averaged f_(esc) over long time-scales generally remains ≲ 5 per cent, considerably lower than the estimate in many reionization models. We find no strong dependence of f_(esc) on galaxy mass or redshift. In our simulations, the intrinsic ionizing photon budgets are dominated by stellar populations younger than 3 Myr, which tend to be buried in dense birth clouds. The escaping photons mostly come from populations between 3 and 10 Myr, whose birth clouds have been largely cleared by stellar feedback. However, these populations only contribute a small fraction of intrinsic ionizing photon budgets according to standard stellar population models. We show that f_(esc) can be boosted to high values, if stellar populations older than 3 Myr produce more ionizing photons than standard stellar population models (as motivated by, e.g. models including binaries). By contrast, runaway stars with velocities suggested by observations can enhance f_(esc) by only a small fraction. We show that ‘sub-grid’ star formation models, which do not explicitly resolve star formation in dense clouds with n ≫ 1 cm^(−3), will dramatically overpredict f_(esc).

Item Type:Article
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URLURL TypeDescription Paper DOIArticle
Hopkins, Philip F.0000-0003-3729-1684
Quataert, Eliot0000-0001-9185-5044
Kereš, Dušan0000-0002-1666-7067
Additional Information:© 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2015 July 22. Received 2015 June 28; in original form 2015 March 24. First published online August 22, 2015. We thank the anonymous referee for a detailed report and helpful suggestions. The simulations used in this paper were run on XSEDE computational resources (allocations TG-AST120025, TG-AST130039, and TG-AST140023). The radiative transfer calculations were run on the Caltech compute cluster ‘Zwicky’ (NSF MRI award PHY-0960291). DK is supported in part by a Department of Energy Office of Nuclear Physics Early Career Award, and by the Director, Office of Energy Research, Office of High Energy and Nuclear Physics, Divisions of Nuclear Physics, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 and by the NSF through grant AST-1109896. Support for PFH was provided by the Gordon and Betty Moore Foundation through Grant 776 to the Caltech Moore Center for Theoretical Cosmology and Physics, by the Alfred P. Sloan Foundation through Sloan Research Fellowship BR2014-022, and by NSF through grant AST-1411920. CAFG was supported by NSF through grant AST-1412836, by NASA through grant NNX15AB22G, and by Northwestern University funds. DK was supported by NSF grant AST-1412153 and funds from the University of California, San Diego. EQ was supported by NASA ATP grant 12-APT12-0183, a Simons Investigator award from the Simons Foundation, the David and Lucile Packard Foundation, and the Thomas Alison Schneider Chair in Physics at UC Berkeley.
Group:TAPIR, Moore Center for Theoretical Cosmology and Physics
Funding AgencyGrant Number
Department of Energy (DOE)DE-AC02-05CH11231
Gordon and Betty Moore Foundation776
Alfred P. Sloan FoundationBR2014-022
Northwestern UniversityUNSPECIFIED
University of California, San DiegoUNSPECIFIED
Simons FoundationUNSPECIFIED
David and Lucile Packard FoundationUNSPECIFIED
University of California, BerkeleyUNSPECIFIED
Subject Keywords:galaxies: evolution – galaxies: formation – galaxies: high-redshift – cosmology: theory
Record Number:CaltechAUTHORS:20150330-163106550
Persistent URL:
Official Citation:Xiangcheng Ma, Daniel Kasen, Philip F. Hopkins, Claude-André Faucher-Giguère, Eliot Quataert, Dušan Kereš, and Norman Murray The difficulty of getting high escape fractions of ionizing photons from high-redshift galaxies: a view from the FIRE cosmological simulations MNRAS (October 11, 2015) Vol. 453 960-975 doi:10.1093/mnras/stv1679 First published online August 22, 2015
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:56223
Deposited By: Joy Painter
Deposited On:30 Mar 2015 23:50
Last Modified:17 Aug 2017 19:43

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