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

Abstract

Reionisation in the early Universe is likely driven by dwarf galaxies. Using\ncosmological radiation-hydrodynamic simulations, we study star formation and\nthe escape of Lyman continuum (LyC) photons from mini-haloes with $M_{\\rm halo}\n\\le 10^8\\,M_\\odot$. Our simulations include a new thermo-turbulent star\nformation model, non-equilibrium chemistry, and relevant stellar feedback\nprocesses (photoionisation by young massive stars, radiation pressure, and\nmechanical supernova explosions). We find that feedback reduces star formation\nvery efficiently in mini-haloes, resulting in the stellar mass consistent with\nthe slope and normalisation reported in Kimm \\& Cen and the empirical stellar\nmass-to-halo mass relation derived in the local Universe. Because star\nformation is stochastic and dominated by a few gas clumps, the escape fraction\nin mini-haloes is generally determined by radiation feedback (heating due to\nphoto-ionisation), rather than supernova explosions. We also find that the\nphoton number-weighted mean escape fraction in mini-haloes is higher\n($\\sim20$-$40\\%$) than that in atomic-cooling haloes, although the\ninstantaneous fraction in individual haloes varies significantly. The escape\nfraction from Pop III stars is found to be significant ($\\ge10\\%$) only when\nthe mass is greater than $\\sim$100\\,\\msun. Based on simple analytic\ncalculations, we show that LyC photons from mini-haloes are, despite their high\nescape fractions, of minor importance for reionisation due to inefficient star\nformation. We confirm previous claims that stars in atomic-cooling haloes with\nmasses $10^8\\,M_\\odot\\le M_{\\rm halo} \\le 10^{11}\\,M_\\odot$ are likely to be\nthe most important source of reionisation.\n