Escape Fraction of Ionizing Radiation from Starburst Galaxies at High Redshifts

TL;DR

This paper models the evolution of ionization fronts in early galaxies to understand how the escape fraction of ionizing radiation changes over cosmic time, especially during reionization, highlighting the role of low-mass halos.

Contribution

It provides a theoretical calculation of how the escape fraction varies with galaxy mass and redshift, emphasizing the decline of escape fraction due to feedback effects during reionization.

Findings

High escape fraction in low-mass halos with M<10^8.7 Msun at z=10

Escape fraction decreases over time during reionization

Radiative feedback depletes low-mass galaxy population

Abstract

Recent data indicates that the cosmic UV emissivity decreased with decreasing redshift z near the end of reionization. Lacking evidence for very massive early stars, this could signal a decline with time in the mass-averaged escape fraction of ionizing radiation from galaxies <fesc> at z > 6. We calculate the evolution of ionization fronts in dark matter halos which host gas in hydrostatic equilibrium at its cooling temperature floor (T~10^4 K for atomic hydrogen). We find a high escape fraction only for the lowest mass halos (with M< 10^8.7 Msun at (1+z)=10) provided their star formation efficiency f_star > 10^-3. Since the low-mass galaxy population is depleted by radiative feedback, we find that indeed <fesc> decreases with time during reionization.