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 _ { \mathrm { esc } } ) .
Here we update this picture via an empirical model that successfully predicts latest observations ( e.g. , the rapid drop in star-formation density ( \rho _ { \mathrm { SFR } } ) at z > 8 ) .
We generate an ionizing spectrum for each galaxy in our model and constrain f _ { \mathrm { esc } } by leveraging latest measurements of the reionization timeline ( e.g. , Ly \alpha damping of quasars and galaxies at z > 7 ) .
Assuming a constant f _ { \mathrm { esc } } across all sources at z > 6 , we find M _ { \mathrm { UV } } < -13.5 galaxies need f _ { \mathrm { 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 rapidly in 300 Myrs , driven by the z > 8 \rho _ { \mathrm { SFR } } and favored by high neutral fractions ( \sim 60 - 90 \% ) measured at z \sim 7 - 8 .
Inspired by the emergent sample of Lyman Continuum ( LyC ) leakers spanning z \sim 0 - 6.6 that overwhelmingly displays higher-than-average star-formation surface density ( \Sigma _ { \mathrm { SFR } } ) , we propose a physically motivated model relating f _ { \mathrm { esc } } to \Sigma _ { \mathrm { SFR } } and find f _ { \mathrm { esc } } \propto \Sigma ^ { 0.4 \pm 0.1 } _ { \mathrm { SFR } } .
Since \Sigma _ { \mathrm { SFR } } falls by \sim 2.5 dex between z = 8 and z = 0 , our model explains the humble upper limits on f _ { \mathrm { esc } } at lower redshifts and its required evolution to f _ { \mathrm { esc } } \sim 0.2 at z > 6 .
Within this model , strikingly , < 5 \% of galaxies with M _ { \mathrm { UV } } < -18 and \log ( M _ { \star } / M _ { \odot } ) > 8 ( the ‘ oligarchs ’ ) account for \gtrsim 80 \% of the reionization budget – a stark departure from the canonical ‘ democratic ’ reionization led by copious faint sources .
In fact , faint sources ( M _ { \mathrm { 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 ( \alpha _ { \mathrm { UV } } > -2 ) and/or f _ { \mathrm { esc } } distributions skewed toward massive galaxies produce the required late and rapid reionization .
We predict LyC leakers like COLA1 ( z = 6.6 , f _ { \mathrm { esc } } \sim 30 \% , M _ { \mathrm { UV } } = -21.5 ) become increasingly common towards z \sim 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 .