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  • Rapid Reionization by the O...
    Naidu, Rohan P.; Tacchella, Sandro; Mason, Charlotte A.; Bose, Sownak; Oesch, Pascal A.; Conroy, Charlie

    The Astrophysical journal, 04/2020, Letnik: 892, Številka: 2
    Journal Article

    The protagonists of the last great phase transition of the universe-cosmic reionization-remain elusive. Faint star-forming galaxies are leading candidates because they are found to be numerous and may have significant ionizing photon escape fractions ( ). Here we update this picture via an empirical model that successfully predicts latest observations (e.g., the rapid drop in star-formation density ( at ). We generate an ionizing spectrum for each galaxy in our model and constrain by leveraging latest measurements of the reionization timeline (e.g., Ly damping of quasars and galaxies at z > 7). Assuming a constant across all sources at z > 6, we find < −13.5 galaxies need = to complete reionization. The inferred Intergalactic Medium neutral fraction is 0.9, 0.5, 0.1 at -that is, the bulk of reionization transpires rapidly in 300 Myr, driven by the z > 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 ( ), we propose a physically motivated model relating to and find . Since falls by ∼2.5 dex between z = 8 and z = 0, our model explains the humble upper limits on at lower redshifts and its required evolution to ∼ 0.2 at z > 6. Within this model, strikingly, <5% of galaxies with < −18 and log(M /M ) > 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 ( > −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 ( > −2) and/or distributions skewed toward massive galaxies produce the required late and rapid reionization. We predict that LyC leakers like COLA1 (z = 6.6, ∼ 30%, = −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.