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 $M_{UV}$<-13.5
galaxies need $f_{esc}=0.21^{+0.06}_{-0.04}$ to complete reionization. The
inferred IGM neutral fraction is [0.9, 0.5, 0.1] at z=[8.2, 6.8, 6.2]$\pm$0.2,
i.e., the bulk of reionization transpires in 300 Myrs. Inspired by the emergent
sample of Lyman Continuum (LyC) leakers that overwhelmingly displays
higher-than-average star-formation surface density ($\Sigma$), we propose a
model relating $f_{esc}$ to $\Sigma$ and find
$f_{esc}\propto\Sigma^{0.4\pm0.1}$. Since $\Sigma$ 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 ~0.2 at z>6. Within this model,
strikingly, <5% of galaxies with $M_{UV}$<-18 (the `oligarchs') account for
>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)
