The escape of ionising radiation from high-redshift dwarf galaxies

Abstract

Context. The UV escape fraction from high-redshift galaxies plays a key role in models of cosmic reionisation. Because it is currently not possible to deduce the escape fractions during the epoch of reionisation from observations, we have to rely on numerical simulations. Aims. We aim to better constrain the escape fraction from high-redshift dwarf galaxies, as these are the most likely sources responsible for reionising the Universe. Methods. We employ a N-body/SPH method that includes realistic prescriptions for the physical processes that are important for the evolution of dwarf galaxies. These models are post-processed with radiative transfer to determine the escape fraction of ionising radiation. We perform a parameter study to assess the influence of the spin parameter, gas fraction and formation redshift of the galaxy and study the importance of numerical parameters as resolution, source distribution and local gas clearing. Results. We find that the UV escape fraction from high-redshift dwarf galaxies that have formed a rotationally supported disc lie between 10-5 and 0.1. The mass and angular momentum of the galaxy are the most important parameters that determine the escape fraction. We compare our results to previous work and discuss the uncertainties of our models. Conclusions. The low escape fraction we find for high-redshift dwarf galaxies is balanced by their high stellar content, resulting in an efficiency parameter for stars that is only marginally lower than the values found by semi-analytic models of reionisation. We therefore conclude that dwarf galaxies play an important role in cosmic reionisation also after the initial starburst phase, when the gas has settled into a disc.