Rapid Reionization by the Oligarchs: The Case for Massive, UV-Bright, Star-Forming Galaxies with High Escape Fractions

The protagonists of cosmic reionization remain elusive. Faint star-forming galaxies are leading candidates because they are 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 drop in star-formation density at z>8). We generate an ionizing spectrum for each galaxy in our model and constrain $f_{esc}$ using latest measurements of the reionization timeline (e.g., Ly$\alpha$ damping of quasars and galaxies at z>7). Assuming a constant $f_{esc}$, we find $M_{UV}$ 6. Within this model, strikingly, 80% of the reionization budget. In fact, faint sources ($M_{UV}$>-16) must be relegated to a limited role to ensure high neutral fractions at z=7-8. Shallow faint-end slopes of the UV luminosity function ($\alpha$>-2) and/or $f_{esc}$ distributions skewed toward bright galaxies produce the required late and rapid reionization. We predict LyC leakers like COLA1 (z=6.6, $f_{esc}$~30%, $M_{UV}$=-21.5) become increasingly common towards 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. (abridged)