TY - JOUR

T1 - Rapid Reionization by the Oligarchs

T2 - The Case for Massive, UV-bright, Star-forming Galaxies with High Escape Fractions

AU - Naidu, Rohan P.

AU - Tacchella, Sandro

AU - Mason, Charlotte A.

AU - Bose, Sownak

AU - Oesch, Pascal A.

AU - Conroy, Charlie

PY - 2020/4/1

Y1 - 2020/4/1

N2 - The protagonists of the last great phase transition of the universe-cosmic reionization-remain elusive. Faint starforming galaxies are leading candidates because they are found to be numerous and may have significant ionizing photon escape fractions ( fesc). Here we update this picture via an empirical model that successfully predicts latest observations (e.g., the rapid drop in star-formation density (rSFR at z > 8). We generate an ionizing spectrum for each galaxy in our model and constrain fesc by leveraging latest measurements of the reionization timeline (e.g., Lya damping of quasars and galaxies at z.>.7). Assuming a constant fesc across all sources at z.>.6, we find MUV..8.SFR and favored by high neutral fractions (60%-90%) measured at z..7-8. Inspired by the emergent sample of Lyman Continuum (LyC) leakers spanning z..0-6.6 that overwhelmingly displays higher-than-average star-formation surface density (SSFR), we propose a physically motivated model relating fesc to SSFR and find fesc mu S. SFR0.4 0.1. Since SSFR falls by 2.5 dex between z.=.8 and z.=.0, our model explains the humble upper limits on fesc at lower redshifts and its required evolution to fesc..0.2 at z.>.6. Within this model, strikingly, .8 (the "oligarchs") account for..80% of the reionization budget-a stark departure from the canonical "democratic" reionization led by copious faint sources. In fact, faint sources (MUV.>. -16) must be relegated to a limited role in order to ensure high neutral fractions at z.=.7-8. Shallow faint-end slopes of the UV luminosity function (aUV.>.-2) and/or fesc distributions skewed toward massive galaxies produce the required late and rapid reionization. We predict that LyC leakers like COLA1 (z.=.6.6, fesc..30%, MUV.=.-21.5) will become increasingly common toward z..6 and that the drivers of reionization do not lie hidden across the faint end of the luminosity function but are already known to us.

AB - The protagonists of the last great phase transition of the universe-cosmic reionization-remain elusive. Faint starforming galaxies are leading candidates because they are found to be numerous and may have significant ionizing photon escape fractions ( fesc). Here we update this picture via an empirical model that successfully predicts latest observations (e.g., the rapid drop in star-formation density (rSFR at z > 8). We generate an ionizing spectrum for each galaxy in our model and constrain fesc by leveraging latest measurements of the reionization timeline (e.g., Lya damping of quasars and galaxies at z.>.7). Assuming a constant fesc across all sources at z.>.6, we find MUV..8.SFR and favored by high neutral fractions (60%-90%) measured at z..7-8. Inspired by the emergent sample of Lyman Continuum (LyC) leakers spanning z..0-6.6 that overwhelmingly displays higher-than-average star-formation surface density (SSFR), we propose a physically motivated model relating fesc to SSFR and find fesc mu S. SFR0.4 0.1. Since SSFR falls by 2.5 dex between z.=.8 and z.=.0, our model explains the humble upper limits on fesc at lower redshifts and its required evolution to fesc..0.2 at z.>.6. Within this model, strikingly, .8 (the "oligarchs") account for..80% of the reionization budget-a stark departure from the canonical "democratic" reionization led by copious faint sources. In fact, faint sources (MUV.>. -16) must be relegated to a limited role in order to ensure high neutral fractions at z.=.7-8. Shallow faint-end slopes of the UV luminosity function (aUV.>.-2) and/or fesc distributions skewed toward massive galaxies produce the required late and rapid reionization. We predict that LyC leakers like COLA1 (z.=.6.6, fesc..30%, MUV.=.-21.5) will become increasingly common toward z..6 and that the drivers of reionization do not lie hidden across the faint end of the luminosity function but are already known to us.

KW - O III EMITTERS

KW - SIMILAR-TO 3

KW - LYMAN-CONTINUUM

KW - LUMINOSITY FUNCTION

KW - IONIZING PHOTONS

KW - INTERGALACTIC MEDIUM

KW - CENT LEAKAGE

KW - FAINT END

KW - REDSHIFT

KW - EVOLUTION

U2 - 10.3847/1538-4357/ab7cc9

DO - 10.3847/1538-4357/ab7cc9

M3 - Journal article

VL - 892

JO - Astrophysical Journal

JF - Astrophysical Journal

SN - 0004-637X

IS - 2

M1 - 109

ER -