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 (f_esc). Here we update this picture via an empirical model that successfully predicts latest observations (e.g., the rapid drop in star-formation density (ρ_SFR at z > 8). We generate an ionizing spectrum for each galaxy in our model and constrain f_esc by leveraging latest measurements of the reionization timeline (e.g., Lyα damping of quasars and galaxies at z > 7). Assuming a constant f_esc across all sources at z > 6, we find M_UV < -13.5 galaxies need f_esc = 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 ρ_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 (Σ_SFR), we propose a physically motivated model relating f_esc to Σ_SFR and find f_esc ∝ Σ_SFR^0.4±0.1. Since Σ_SFR falls by ∼2.5 dex between z = 8 and z = 0, our model explains the humble upper limits on f_esc at lower redshifts and its required evolution to f_esc ∼ 0.2 at z > 6. Within this model, strikingly, <5% of galaxies with M_UV < -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 (M_UV > -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 (α_UV > -2) and/or f_esc distributions skewed toward massive galaxies produce the required late and rapid reionization. We predict that LyC leakers like COLA1 (z = 6.6, f_esc ∼ 30%, M_UV = -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.