From oligarchic reionization to the formation of bulges

The protagonists of the last great phase transition of the universe – cosmic reionization – remain elusive. Galaxies at z~2 are already mature: bulges have already formed in the cores of these galaxies. The high stellar mass densities in these systems require a high star-formation rate surface density at earlier times; providing possible candidates of galaxies that drive reionization. Using an empirical model for galaxy evolution at z>4, we constrain the escape fraction by leveraging latest constraints on the reionization timeline, such as the Ly-alpha damping of quasars and galaxies at z>7. 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 f_esc to star-formation surface density. Within this model, strikingly, < 5% of galaxies with M_UV<-18 and log(M)>8 (the “oligarchs”) account for >80% of the reionization budget – a stark departure from the canonical “democratic” reionization led by copious faint sources. We predict that the drivers of reionization do not lie hidden across the faint-end of the luminosity function, but are already known to us.