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 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 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.