Too small to succeed: the difficulty of sustaining star formation in low-mass haloes

We present high-resolution simulations of an isolated dwarf spheroidal (dSph) galaxy between redshifts z ∼ 10 and z ∼ 4, the epoch when several Milky Way dSph satellites experienced extended star formation, in order to understand in detail the physical processes which affect a low-mass halo's ability to retain gas. It is well established that supernova feedback is very effective at expelling gas from a 3 × 10$^{7}$ M⊙ halo, the mass of a typical redshift 10 progenitor of a redshift 0 halo with mass ∼10$^{9}$ M⊙. We investigate the conditions under which such a halo is able to retain sufficient high-density gas to support extended star formation. In particular, we explore the effects of: an increased relative concentration of the gas compared to the dark matter; a higher concentration dark matter halo; significantly lower supernova rates; enhanced metal cooling due to enrichment from earlier supernovae. We show that disc-like gas distributions retain more gas than spherical ones, primarily due to the shorter gas cooling times in the disc. However, a significant reduction in the number of supernovae compared to that expected for a standard initial mass function is still needed to allow the retention of high-density gas. We conclude that the progenitors of the observed dSphs would only have retained the gas required to sustain star formation if their mass, concentration and gas morphology were already unusual for those of a dSph-mass halo progenitor by a redshift of 10.CRC and MAB are supported by a Science and Technology facilities council (STFC) PhD studentship. CP acknowledges support from the Australian Research Council (ARC) Future Fellowship FT130100041 and Discovery projects DP130100117 and DP140100198. Figs 4 and 6 were produced using SPLASH (Price 2013). 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