On the origin of low escape fractions of ionizing radiation from massive star-forming galaxies at high redshift

The physical origin of low escape fractions of ionizing radiation derived from massive star-forming galaxies at z similar to 3-4 is not well understood. We perform idealized disc galaxy simulations to understand how galactic properties such as metallicity and gas mass affect the escape of Lyman continuum (LyC) photons using radiation-hydrodynamic simulations with strong stellar feedback. We find that the luminosity-weighted escape fraction from a metal-poor (Z= 0.002) galaxy embedded in a halo ofmass M-h similar or equal to 10(11) M-circle dot is < f(esc)(3D) > similar or equal to 10 per cent. Roughly half of the LyC photons are absorbed within scales of 100 pc, and the other half is absorbed in the ISM (less than or similar to 2 kpc). When the metallicity of the gas is increased to Z = 0.02, the escape fraction is significantly reduced to < f(esc)(3D) > similar or equal to 1 per cent because young stars are enshrouded by their birth clouds for a longer time. In contrast, increasing the gas mass by a factor of 5 leads to < f(esc)(3D) > similar or equal to 5 per cent because LyC photons are only moderately absorbed by the thicker disc. Our experiments suggest that high metallicity is likely more responsible for the low escape fractions observed in massive star-forming galaxies, supporting the scenario in which the escape fraction is decreasing with increasing halo mass. Finally, negligible correlation is observed between the escape fraction and surface density of star formation or galactic outflow rates.