Runaway Stars and the Escape of Ionizing Radiation from High-Redshift Galaxies

Authors: Conroy & Kratter
Paper: here

In this paper, the effect of runaway stars on the escape fraction of ionising radiation of galaxies around redshift 10 (so during the epoch of reionisation) is investigated. Runaway stars are stars with high velocities that are able to migrate from the dense environment in which they were formed. This would significantly increase the fraction of photons produced by these stars that is able to escape into the IGM. The two main formation channels are through dynamical ejections from young stellar clusters and through the explosion of companion star. The first channel is most important, since the stars that produce most ionising photons are short-lived. The fraction of massive stars that are runaways is highly uncertain, ranging from 10% to 50%. The authors adopt a value of 30%.

To assess the enhancement of the escape fraction due to runaway stars the authors adopt a simple analytic model for the galaxies, similar to Ricotti & Shull (2000). The two models for the gas distribution are a spherical (exponential) number density profile and an isothermal disc profile. The escape fraction is computed by considering the distance and line-of-sight column density from the position of the star to the virial radius, integrated over all solid angles. The distribution of the stars follows that of the gas.

With this simple model, the authors find that for galaxies at redshift 10, the escape fraction can be higher up to a factor of 7 when runaway stars are included. The reason is that the non-runaway stars have escape fractions of less than 10% due to the dense environment of the galaxy. The enhancement peaks around a halo mass of 1e8 solar masses, because runaways travel a fixed distance inside the galaxy and higher mass galaxies have larger radii.

The authors discuss the effects of various assumptions in their model, by changing the redshift, the dust attenuation, the minimum and maximum stellar mass, the runaway velocity, the ionising luminosity function and the gas density profile. The latter had the most significant effect, with enhancement scaling different with halo mass for spherical and disc-like galaxies.

This study provides a rough first estimate of what the effect of runaway stars on the escape fraction could be, but, as the authors show, the significant dependence on the gas distribution makes the conclusions not very robust.