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

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.

Cosmic reionization after Planck

“…More recently, using a high-resolution cosmological zoom-in simulation of galaxy formation, Ma et al (2015) found a considerably lower (< 5%; much less than required by reionization models) and a non-evolving escape fraction. This unchanging trend of fesc is as well consistent with many other earlier results (Gnedin 2008;Yajima et al 2011). A decreasing tendency of escape fraction with redshift has also been reported in the literature (Wood & Loeb 2000;Kimm & Cen 2014).…”

“…• From the match between the observed high-redshift LFs and our model predictions, we find that the best-fit values for both ǫ * and fesc remains somewhat constant with redshifts: ǫ * at ∼ 4% and fesc ∼ 10% for z = 6 − 9 (Gnedin 2008;Yajima et al 2011;Ma et al 2015). We have also obtained the tightest constraint available to our knowledge on on fesc (> 10%) at z = 10.…”

Reionization and Galaxy Formation in Warm Dark Matter Cosmologies

“…Finally, we have made the simplifying assumption of using a mass-independent f esc at all z. This is partly motivated by the uncertainty regarding the mass-dependence of f esc : while some authors find f esc to decrease with an increase in the halo mass (Razoumov & Sommer-Larsen 2010;Yajima et al 2011;Ferrara & Loeb 2013) …”

Dark-ages reionization and galaxy formation simulation – III. Modelling galaxy formation and the epoch of reionization

“…However, the value of f esc for an individual galaxy is strongly dependent on its chemical, structural, and kinematic properties, and there exists considerable theoretical and observational evidence to suggest that the average f esc does indeed vary with redshift, mass, and/or star formation rate (e.g. Gnedin et al 2008;Wise & Cen 2009;Paardekooper et al 2011;Yajima et al 2011;Kuhlen & Faucher-Giguère 2012;Mitra et al 2013;Paardekooper et al 2015). …”