Ionizing Photon Escape Fractions from High Redshift Dwarf Galaxies

TL;DR

This study uses high-resolution radiation hydrodynamical simulations to analyze ionizing photon escape fractions from high-redshift dwarf galaxies, revealing their significant role in cosmic reionization.

Contribution

It provides detailed simulation-based insights into escape fractions across various dwarf galaxy masses and conditions, highlighting their impact on reionization.

Findings

Escape fractions vary up to a factor of two depending on initial conditions.

Time-averaged escape fractions exceed 25%, reaching 80% in halos above 10^8 solar masses.

Star formation efficiency times escape fraction ranges from 0.02 to 0.03, depending on the IMF.

Abstract

It has been argued that low-luminosity dwarf galaxies are the dominant source of ionizing radiation during cosmological reionization. The fraction of ionizing radiation that escapes into the intergalactic medium from dwarf galaxies with masses less than ~10^9.5 solar masses plays a critical role during this epoch. Using an extensive suite of very high resolution (0.1 pc), adaptive mesh refinement, radiation hydrodynamical simulations of idealized and cosmological dwarf galaxies, we characterize the behavior of the escape fraction in galaxies between 3 x 10^6 and 3 x 10^9 solar masses with different spin parameters, amounts of turbulence, and baryon mass fractions. For a given halo mass, escape fractions can vary up to a factor of two, depending on the initial setup of the idealized halo. In a cosmological setting, we find that the time-averaged photon escape fraction always exceeds 25% and reaches up to 80% in halos with masses above 10^8 solar masses with a top-heavy IMF. The instantaneous escape fraction can vary up to an order of magnitude in a few million years and tend to be positively correlated with star formation rate. We find that the mean of the star formation efficiency times ionizing photon escape fraction, averaged over all atomic cooling (T_vir > 8000 K) galaxies, ranges from 0.02 for a normal IMF to 0.03 for a top-heavy IMF, whereas smaller, molecular cooling galaxies in minihalos do not make a significant contribution to reionizing the universe due to a much lower star formation efficiency. These results provide the physical basis for cosmological reionization by stellar sources, predominately atomic cooling dwarf galaxies.