Escape fraction of ionizing photons from high-redshift galaxies in cosmological SPH simulations: Escape of ionizing photons

Abstract

Combing the three-dimensional radiative transfer (RT) calculation and cosmological smoothed particle hydrodynamics (SPH) simulations, we study the escape fraction of ionizing photons (fesc) of high-redshift galaxies at z= 3–6. Our simulations cover the halo mass range of Mh= 109–1012 M⊙. We post-process several hundred simulated galaxies with the Authentic Radiative Transfer (art) code to study the halo mass dependence of fesc. In this paper, we restrict ourselves to the transfer of stellar radiation from local stellar population in each dark matter halo. We find that the average fesc steeply decreases as the halo mass increases, with a large scatter for the lower-mass haloes. The low-mass haloes with Mh∼ 109 M⊙ have large values of fesc (with an average of ∼0.4), whereas the massive haloes with Mh∼ 1011 M⊙ show small values of fesc (with an average of ∼0.07). This is because in our simulations, the massive haloes show more clumpy structure in gas distribution, and the star-forming regions are embedded inside these clumps, making it more difficult for the ionizing photons to escape. On the other hand, in low-mass haloes, there are often conical regions of highly ionized gas due to the shifted location of young star clusters from the centre of dark matter halo, which allows the ionizing photons to escape more easily than in the high-mass haloes. By counting the number of escaped ionizing photons, we show that the star-forming galaxies can ionize the intergalactic medium at z= 3–6. The main contributor to the ionizing photons is the haloes with Mh≲ 1010 M⊙ owing to their high fesc. The large dispersion in fesc suggests that there may be various sizes of H ii bubbles around the haloes even with the same mass in the early stages of reionization. We also examine the effect of UV background radiation field on fesc using simple, four different treatments of UV background.