The First Billion Years project: the escape fraction of ionizing photons in the epoch of reionization

Abstract

Proto-galaxies forming in low-mass dark matter haloes are thought to provide the majority of ionizing photons needed to reionize the Universe, due to their high escape fractions of ionizing photons. We study how the escape fraction in high-redshift galaxies relates to the physical properties of the halo in which the galaxies form, by computing escape fractions in more than 75000 haloes between redshifts 27 and 6 that were extracted from the First Billion Years project, high-resolution cosmological hydrodynamics simulations of galaxy formation. We find that the main constraint on the escape fraction is the gas column density in a radius of 10 pc around the stellar populations, causing a strong mass dependence of the escape fraction. The lower potential well in haloes with M 200 10 8 M results in low column densities that can be penetrated by radiation from young stars (age < 5 Myr). In haloes with M 200 10 8 M supernova feedback is important, but only ∼ 30% of the haloes in this mass range have an escape fraction higher than 1%. We find a large range of escape fractions in haloes with similar properties, caused by different distributions of the dense gas in the halo. This makes it very hard to predict the escape fraction on the basis of halo properties and results in a highly anisotropic escape fraction. The strong mass dependence, the large spread and the large anisotropy of the escape fraction may strongly affect the topology of reionization and is something current models of cosmic reionization should strive to take into account.

Growth of First Galaxies: Impacts of Star Formation and Stellar Feedback

“…This code was extended and modified to include the treatment of population III (Pop III) star formation, Lyman-Werner feedback, non-equilibrium primordial chemistry, and dust formation/destruction in the First Billion Year (FiBY) project (e.g., Khochfar & Dalla Vecchia et al in prep.). The impacts of these new physical processes on galaxy formation have been studied in a series of papers of the FiBY project (e.g., Johnson et al 2013;Paardekooper et al 2013;Agarwal & Khochfar 2015;Elliott et al 2015). In this work, we do not consider the formation of Pop III stars and non-equilibrium primordial chemistry, because our focus are more massive galaxies after the era of first mini-halos.…”

“…The central question is how the first galaxies formed and evolved to the observed galaxies at z 6. In addition, the first galaxies are also considered to be important contributor of ionizing photons for cosmic reionization (Yajima et al 2009(Yajima et al , 2011(Yajima et al , 2014aPaardekooper et al 2013Paardekooper et al , 2015Wise et al 2014;Kimm & Cen 2014;Robertson et al 2015;Ma et al 2015). Therefore, understanding star formation in first galaxies would also provide indispensable information for the cosmic reionization history.…”

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). …”

First light: exploring the spectra of high-redshift galaxies in the Renaissance Simulations

“…A recent submission by Cullen et al (2017) explores the effect of dust on galaxies in the Gadget First Billion Years Project (Paardekooper et al 2015). Stellar population synthesis is calculated using BPASSv2 (Eldridge & Stanway 2016) and emission lines are calculated using Cloudy.…”