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No missing photons for reionization: moderate ionizing photon escape fractions from the FIRE-2 simulations

Ma, Xiangcheng and Quataert, Eliot and Wetzel, Andrew and Hopkins, Philip F. and Faucher-Giguère, Claude-André and Kereš, Dušan (2020) No missing photons for reionization: moderate ionizing photon escape fractions from the FIRE-2 simulations. Monthly Notices of the Royal Astronomical Society, 498 (2). pp. 2001-2017. ISSN 0035-8711. doi:10.1093/mnras/staa2404.

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We present the escape fraction of hydrogen ionizing photons (f_(esc)) from a sample of 34 high-resolution cosmological zoom-in simulations of galaxies at z ≥ 5 in the Feedback in Realistic Environments project, post-processed with a Monte Carlo radiative transfer code for ionizing radiation. Our sample consists of 8500 haloes in M_(vir) ∼ 10⁸–10¹²M⊙ (M* ∼ 10⁴–10¹⁰M⊙⁠) at z = 5–12. We find the sample average〈f_(esc)〉increases with halo mass for M_(vir) ∼ 10⁸–10^(9.5) M⊙⁠, becomes nearly constant for 10^(9.5)–10¹¹M⊙, and decreases at ≳10¹¹M⊙. Equivalently,〈f_(esc)〉 increases with stellar mass up to M∗ ∼ 10⁸M⊙ and decreases at higher masses. Even applying single-star stellar population synthesis models, we find a moderate〈f_(esc)〉∼ 0.2 for galaxies at M∗ ∼ 10⁸M⊙⁠. Nearly half of the escaped ionizing photons come from stars 1–3 Myr old and the rest from stars 3–10 Myr old. Binaries only have a modest effect, boosting〈f_(esc)〉by ∼25–35 per cent and the number of escaped photons by 60–80 per cent. Most leaked ionizing photons are from vigorously star-forming regions that usually contain a feedback-driven kpc-scale superbubble surrounded by a dense shell. The shell is forming stars while accelerated, so new stars formed earlier in the shell are already inside the shell. Young stars in the bubble and near the edge of the shell can fully ionize some low-column-density paths pre-cleared by feedback, allowing a large fraction of their ionizing photons to escape. The decrease of〈f_(esc)〉at the high-mass end is due to dust attenuation, while at the low-mass end,〈f_(esc)〉decreases owing to inefficient star formation and hence feedback. At fixed mass,〈f_(esc)〉tends to increase with redshift. Although the absolute〈f_(esc)〉does not fully converge with resolution in our simulations, the mass- and redshift-dependence of〈f_(esc)〉 is likely robust. Our simulations produce sufficient ionizing photons for cosmic reionization.

Item Type:Article
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URLURL TypeDescription Paper
Ma, Xiangcheng0000-0001-8091-2349
Quataert, Eliot0000-0001-9185-5044
Wetzel, Andrew0000-0003-0603-8942
Hopkins, Philip F.0000-0003-3729-1684
Faucher-Giguère, Claude-André0000-0002-4900-6628
Kereš, Dušan0000-0002-1666-7067
Additional Information:© 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Accepted 2020 August 7. Received 2020 August 2; in original form 2020 March 13. Published: 02 September 2020. The simulations and post-processing calculations used in this paper were run on XSEDE computational resources (allocations TG-AST120025, TG-AST130039, TG-AST140023, TG-AST140064, and TG-AST190028). This work was supported in part by a Simons Investigator Award from the Simons Foundation (EQ) and by NSF grant AST-1715070. AW was supported by NASA, through ATP grant 80NSSC18K1097 and HST grants GO-14734 and AR-15057 from STScI. PFH was supported by an Alfred P. Sloan Foundation Research Fellowship, NASA ATP Grant NNX14AH35G, and NSF Collaborative Research Grant #1411920 and CAREER grant #1455342. CAFG was supported by NSF through grants AST-1517491, AST-1715216, and CAREER award AST-1652522, by NASA through grant 17-ATP17-0067, and by a Cottrell Scholar Award from the Research Corporation for Science Advancement. DK was supported by NSF grant AST-1715101 and the Cottrell Scholar Award from the Research Corporation for Science Advancement. Data Availability Statement: The data underlying this article will be shared on reasonable request to the corresponding author.
Group:Astronomy Department, TAPIR
Funding AgencyGrant Number
Simons FoundationUNSPECIFIED
Alfred P. Sloan FoundationUNSPECIFIED
Cottrell Scholar of Research CorporationUNSPECIFIED
Subject Keywords:galaxies: evolution – galaxies: formation – galaxies: high-redshift – cosmology: theory – dark ages, reionization, first stars
Issue or Number:2
Record Number:CaltechAUTHORS:20200626-150134129
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Official Citation:Xiangcheng Ma, Eliot Quataert, Andrew Wetzel, Philip F Hopkins, Claude-André Faucher-Giguère, Dušan Kereš, No missing photons for reionization: moderate ionizing photon escape fractions from the FIRE-2 simulations, Monthly Notices of the Royal Astronomical Society, Volume 498, Issue 2, October 2020, Pages 2001–2017,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:104104
Deposited By: George Porter
Deposited On:26 Jun 2020 22:46
Last Modified:16 Nov 2021 18:28

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