The sources that reionized the universe are still unknown , but likely candidates are faint but numerous galaxies .
In this paper we present results from running a high resolution , uniform volume simulation , the Vulcan , to predict the number densities of undetectable , faint galaxies and their escape fractions of ionizing radiation , f _ { \mathrm { esc } } , during reionization .
Our approach combines a high spatial resolution , a realistic treatment of feedback and hydro processes , a strict threshold for minimum number of resolution elements per galaxy , and a converged measurement of f _ { \mathrm { esc } } .
We calibrate our physical model using a novel approach to create realistic galaxies at z = 0 , so the simulation is predictive at high redshifts .
With this approach we can ( 1 ) robustly predict the evolution of the galaxy UV luminosity function at faint magnitudes down to M _ { \mathrm { UV } } \sim - 15 , two magnitudes fainter than observations , and ( 2 ) estimate f _ { \mathrm { esc } } over a large range of galaxy masses based on the detailed stellar and gas distributions in resolved galaxies .
We find steep faint end slopes , implying high number densities of faint galaxies , and the dependence of f _ { \mathrm { esc } } on the UV magnitude of a galaxy , given by the power-law : \mathrm { log } f _ { \mathrm { esc } } = ( 0.51 \pm 0.04 ) M _ { \mathrm { UV } } +7.3 \pm 0.8 , with the faint population having f _ { \mathrm { esc } } \sim 35 \% .
Convolving the UV luminosity function with f _ { \mathrm { esc } } ( M _ { \mathrm { UV } } ) , we find an ionizing emissivity that is ( 1 ) dominated by the faintest galaxies and ( 2 ) reionizes the universe at the appropriate rate , consistent with observational constraints of the ionizing emissivity and the optical depth to the decoupling surface \tau _ { es } , without the need for additional sources of ionizing radiation .