Efficacy of early stellar feedback in low gas surface density environments

Abstract

We present a suite of high resolution radiation hydrodynamic simulations of a small patch ($1 \\ {\\rm kpc}^2$) of the inter-stellar medium (ISM) performed with Arepo-RT, with the aim to quantify the efficacy of various feedback processes like supernovae explosions (SNe), photoheating and radiation pressure in low gas surface density galaxies ($\\Sigma_{\\rm gas} \\simeq 10 \\ {\\rm M}_\\odot \\ {\\rm pc}^{-2}$). We show that radiation fields decrease the star formation rate and therefore the total stellar mass formed by a factor of $\\sim 2$. This increases the gas depletion timescale and brings the simulated Kennicutt-Schmidt relation closer to the observational estimates. Radiation feedback coupled with SNe is more efficient at driving outflows with the mass and energy loading increasing by a factor of $\\sim 10$. This increase is mainly driven by the additional entrainment of medium density ($10^{-2} \\leq n< 1 \\ {\\rm cm}^{-3}$), warm ($300 \\ {\\rm K}\\leq T<8000 \\ {\\rm K}$) material. Therefore including radiation fields tends to launch colder, denser and higher mass and energy loaded outflows. This is because photoheating of the high density gas around a newly formed star over-pressurises the region, causing it to expand. This reduces the ambient density in which the SNe explode by a factor of $10-100$ which in turn increases their momentum output by a factor of $\\sim 1.5-2.5$. Finally, we note that in these low gas surface density environments, radiation fields primarily impact the ISM via photoheating and radiation pressure has only a minimal role in regulating star formation.