Efficient Simulations of Early Structure Formation and Reionization

Abstract

Detailed theoretical studies of the high-redshift universe, and especially reionization, are generally forced to rely on time-consuming N-body codes and/or radiative transfer algorithms. We present a method to construct seminumerical ''simulations,'' which can efficiently generate realizations of halo distributions and ionization maps at high redshifts. Our procedure combines an excursion-set approach with first-order Lagrangian perturbation theory and operates directly on the linear density and velocity fields. As such, the achievable dynamic range with our algorithm surpasses the current practical limit of N-body codes by orders of magnitude. This is particularly significant in studies of reionization, where the dynamic range is the principal limiting factor, because ionized regions reach scales of tens of comoving Mpc. We test our halo-finding and ionization-mapping algorithms separately against N-body simulations with radiative transfer and obtain excellent agreement. We compute the size distributions of ionized and neutral regions in our maps. We find even larger ionized bubbles than do purely analytic models at the same volume-weighted mean hydrogen neutral fraction,x H i , especially early in reionization. We also generate maps and power spectra of 21 cm brightness temperature fluctuations, which for the first time include corrections due to gas bulk velocities. We find that velocities widen the tails of the temperature distributions and increase small-scale power, although these effects quickly diminish as reionization progresses. We also include some preliminary results from a simulation run with the largest dynamic range to date: a 250 Mpc box that resolves halos with masses M ! 2:2 ; 10 8 M . We show that accurately modeling the late stages of reionization,x H i P 0:5, requires such large scales. The speed and dynamic range provided by our seminumerical approach will be extremely useful in the modeling of early structure formation and reionization.

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

“…The ionizing emissivity of the sources is generally taken to scale with the mass of the halo (e.g. Trac & Cen 2007;Mesinger & Furlanetto 2007;Iliev et al 2012). At first sight this seems a reasonable guess because the star formation rate is found to scale with halo mass, so haloes with a higher mass produce more ionizing photons.…”

Cross-correlation of the cosmic 21-cm signal and Lyman α emitters during reionization

“…We vary both the EoR morphology and the average halo masses, as shown in the figure labels. The average scale of HII regions, computed with the Monte-Carlo mean free path approach of Mesinger & Furlanetto (2007), are denoted with vertical ticks on the panels.…”

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

“…A radiative transfer method (for example ray-tracing algorithms) can then be used to calculate the ionization structure on large scales. In recent years, new hybrid, or semi-numerical models (Mesinger & Furlanetto 2007;Geil & Wyithe 2008;Kim et al 2013a) have been developed that combine N -body simulations with analytical methods to enable the calculation of reionization structure in very large volumes with high efficiency. These methods have elucidated the primary features of the ionization structure during reionization, but do not capture the physics of galaxy formation.…”