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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 (f esc ). Here we update this picture via an empirical model that successfully predicts latest observations (e.g., the rapid drop in star-formation density (ρ SFR ) at z > 8). We generate an ionizing spectrum for each galaxy in our model and constrain f esc by leveraging latest measurements of the reionization timeline (e.g., Lyα damping of quasars and galaxies at z > 7). Assuming a constant f esc across all sources at z > 6, 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] ± 0.2, i.e., the bulk of reionization transpires rapidly in 300 Myrs, 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 (Σ SFR ), we propose a physically motivated model relating f esc to Σ SFR and find f esc ∝ Σ 0.4±0.1 SFR . Since Σ SFR 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 f esc ∼ 0.2 at z > 6. Within this model, strikingly, <5% of galaxies with M UV < −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 (M UV >−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 (α UV > −2) and/or f esc distributions skewed toward massive 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.

We explore the connection between the UV luminosity functions (LFs) of high-z galaxies and the distribution of stellar masses and star-formation histories (SFHs) in their host dark matter halos. We provide a baseline for a redshiftindependent star-formation efficiency model to which observations and models can be compared. Our model assigns a star-formation rate (SFR) to each dark matter halo based on the growth rate of the halo and a redshift-independent star-formation efficiency. The dark matter halo accretion rate is obtained from a high-resolution N -body simulation in order to capture the stochasticity in accretion histories and to obtain spatial information for the distribution of galaxies. The halo mass dependence of the star-formation efficiency is calibrated at z = 4 by requiring a match to the observed UV LF at this redshift. The model then correctly predicts the observed UV LF at z = 5 − 10. We present predictions for the UV luminosity and stellar mass functions, JWST number counts, and SFHs. In particular, we find a stellar-to-halo mass relation at z = 4 − 10 that scales with halo mass at M h < 10 11 M as M ∝ M 2 h , with a normalization that is higher than the relation inferred at z = 0. The average SFRs increase as a function of time to z = 4, although there is significant scatter around the average: about 6% of the z = 4 galaxies show no significant mass growth. Using these SFHs, we present redshift-dependent UV-to-SFR conversion factors, mass return fractions, and mass-to-light ratios for different intial mass functions and metallicities, finding that current estimates of the cosmic SFR density at z ∼ 10 may be overestimated by ∼ 0.1 − 0.2 dex.

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