Rapid Reionization by the Oligarchs: The Case for Massive, UV-bright, Star-forming Galaxies with High Escape Fractions
Abstract 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 α 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 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 ( ), 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.