Rapid Reionization by the Oligarchs: The Case for Massive, UV-bright, Star-forming Galaxies with High Escape Fractions
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 (fesc). Here we update this picture via an empirical model that successfully predicts latest observations (e.g., the rapid drop in star-formation density (ρSFR 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., Lyα damping of quasars and galaxies at z > 7). Assuming a constant fesc across all sources at z > 6, we find MUV < -13.5 galaxies need fesc = 0.21-0.04+0.06 to complete reionization. The inferred Intergalactic Medium neutral fraction is [0.9, 0.5, 0.1] at z=[8.2,6.8,6.2] ± 0.2 - that is, the bulk of reionization transpires rapidly in 300 Myr, driven by the z > 8 ρSFR 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 (ΣSFR), we propose a physically motivated model relating fesc to ΣSFR and find fesc ∝ ΣSFR0.4±0.1. Since ΣSFR 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, <5% of galaxies with MUV < -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 (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 (αUV > -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.