Lyman-Werner UV Escape Fractions from Primordial Halos

TL;DR

This study uses radiation hydrodynamical simulations to quantify Lyman-Werner UV escape fractions from primordial halos, revealing their dependence on halo and stellar mass and highlighting the importance of hydrogen shielding effects.

Contribution

First to parametrize realistic LW escape fractions from primordial halos based on detailed simulations considering H$_2$ and hydrogen shielding effects.

Findings

LW photon escape fractions range from 0% to 85%.

No LW photons escape the most massive halos for the studied stars.

Hydrogen shielding reduces escape fractions by up to a factor of three.

Abstract

Population III stars can regulate star formation in the primordial Universe in several ways. They can ionize nearby halos, and even if their ionizing photons are trapped by their own halos, their Lyman-Werner (LW) photons can still escape and destroy H$_2$ in other halos, preventing them from cooling and forming stars. LW escape fractions are thus a key parameter in cosmological simulations of early reionization and star formation but have not yet been parametrized for realistic halos by halo or stellar mass. To do so, we perform radiation hydrodynamical simulations of LW UV escape from 9--120 M$_{\odot}$ Pop III stars in $10^5$ to $10^7$ M$_{\odot}$ halos with ZEUS-MP. We find that photons in the LW lines (i.e. those responsible for destroying H$_{2}$ in nearby systems) have escape fractions ranging from 0% to 85%. No LW photons escape the most massive halo in our sample, even from the most massive star. Escape fractions for photons elsewhere in the 11.18--13.6~eV energy range, which can be redshifted into the LW lines at cosmological distances, are generally much higher, being above 60% for all but the least massive stars in the most massive halos. We find that shielding of H$_2$ by neutral hydrogen, which has been neglected in most studies to date, produces escape fractions that are up to a factor of three smaller than those predicted by H$_2$ self-shielding alone.