Feedback-regulated star formation and escape of LyC photons from mini-haloes during reionization

Reionization in the early Universe is likely driven by dwarf galaxies. Using cosmological radiation-hydrodynamic simulations, we study star formation and the escape of Lyman continuum (LyC) photons from mini-haloes with $\textit{M}$$_{halo}$ $\lesssim$ 10$^{8}$ M$_{\odot}$. Our simulations include a new thermo-turbulent star formation model, non-equilibrium chemistry and relevant stellar feedback processes (photoionization by young massive stars, radiation pressure and mechanical supernova explosions). We find that feedback reduces star formation very efficiently in mini-haloes, resulting in the stellar mass consistent with the slope and normalization reported in Kimm & Cen and the empirical stellar mass-to-halo mass relation derived in the local Universe. Because star formation is stochastic and dominated by a few gas clumps, the escape fraction in mini-haloes is generally determined by radiation feedback (heating due to photoionization), rather than supernova explosions. We also find that the photon number-weighted mean escape fraction in mini-haloes is higher (~ 20–40 per cent) than that in atomic-cooling haloes, although the instantaneous fraction in individual haloes varies significantly. The escape fraction from Pop III stars is found to be significant ( $\gtrsim$ 10 per cent) only when the mass is greater than ~100 M$\odot$. Based on simple analytic calculations, we show that LyC photons from mini-haloes are, despite their high escape fractions, of minor importance for reionization due to inefficient star formation. We confirm previous claims that stars in atomic-cooling haloes with masses 10$^{8}$ M$_{\odot}$ $\lesssim$ $\textit{M}$$_{halo}$ $\lesssim$ 10$^{11}$ M$_{\odot}$ are likely to be the most important source of reionization.This work was supported by the ERC Advanced Grant 320596 ‘The Emergence of Structure during the Epoch of Reionization’ and the Spin(e) grants ANR-13-BS05-0005 of the French Agence Nationale de la Recherche (http://cosmicorigin.org). HK is supported by Foundation Boustany, the Isaac Newton Studentship and the Cambridge Overseas Trust. JR was funded by the European Research Council under the European Unions Seventh Framework Programme (FP7/2007-2013)/ERC Grant agreement 278594-GasAroundGalaxies, and the Marie Curie Training Network CosmoComp (PITN- GA-2009-238356). JD and AS's research is supported by the funding from Adrian Beecroft, the Oxford Martin School and the STFC. This work used the DiRAC Complexity system, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment is funded by BIS National E-Infrastructure capital grant ST/K000373/1 and STFC DiRAC Operations grant ST/K0003259/1. DiRAC is part of the National E-Infrastructure