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

Combing the three-dimensional radiative transfer (RT) calculation and cosmological smoothed particle hydrodynamics (SPH) simulations, we study the escape fraction of ionizing photons (f_esc) of high-redshift galaxies at z= 3-6. Our simulations cover the halo mass range of M_h= 10⁹-10¹²M_⊙. We post-process several hundred simulated galaxies with the Authentic Radiative Transfer (art) code to study the halo mass dependence of f_esc. 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 f_esc steeply decreases as the halo mass increases, with a large scatter for the lower-mass haloes. The low-mass haloes with M_h∼ 10⁹M_⊙ have large values of f_esc (with an average of ∼0.4), whereas the massive haloes with M_h∼ 10¹¹M_⊙ show small values of f_esc (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 M_h≲ 10¹⁰M_⊙ owing to their high f_esc. The large dispersion in f_esc suggests that there may be various sizes of Hii 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 f_esc using simple, four different treatments of UV background.