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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. . (Unpublished)

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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 halos in M_(vir) ~ 10⁸--10¹² M_⊙ (M_★ ~ 10⁴--10¹⁰ M_⊙) at z = 5--12. We find the sample average <f_(esc)> increases with halo mass for M_(vir) ~ 10⁸--10^(9.5) M_sun, becomes nearly constant for M_vir~10^{9.5}--10^{11} M_⊙, and decreases at M_(vir) > 10¹¹ M_⊙. Equivalently, <f_(esc)> increases with stellar mass up to M_star~10^8 M_sun 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_★ ~ 10⁸ M_⊙. 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% and the number of escaped photons by 60--80%. 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. Our simulations produce sufficient ionizing photons for cosmic reionization.

Item Type:Report or Paper (Discussion Paper)
Related URLs:
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: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 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,¢aAST-1715216, and CAREER award AST-1652522, by NASA through grant¢a17-ATP17-0067,¢aand 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.
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
Record Number:CaltechAUTHORS:20200626-150134129
Persistent URL:
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:26 Jun 2020 22:46

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