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.

We study the impact of ultraviolet background (UVB) radiation field and the local stellar radiation on the H i column density distribution f (N HI ) of damped Lyα systems (DLAs) and sub-DLAs at z = 3 using cosmological smoothed particle hydrodynamics simulations. We find that, in the previous simulations with an optically thin approximation, the UVB was sinking into the H i cloud too deeply, and therefore we underestimated the f (N HI ) at 19 < log N HI < 21.2 compared to the observations. If the UVB is shut off in the high-density regions with n gas > 6 × 10 −3 cm −3 , then we reproduce the observed f (N HI ) at z = 3 very well. We also investigate the effect of local stellar radiation by post-processing our simulation with a radiative transfer code, and find that the local stellar radiation does not change the f (N HI ) very much. Our results show that the shape of f (N HI ) is determined primarily by the UVB with a much weaker effect by the local stellar radiation and that the optically thin approximation often used in cosmological simulation is inadequate to properly treat the ionization structure of neutral gas in and out of DLAs. Our result also indicates that the DLA gas is closely related to the transition region from optically-thick neutral gas to optically-thin ionized gas within dark matter halos.

We study the early stage of the formation of seed supermassive black holes via direct collapse in dark matter (DM) halos, in the cosmological context. We perform highresolution zoom-in simulations of such collapse at high-z. Using the adaptive mesh refinement code ENZO, we resolve the formation and growth of a DM halo, until its virial temperature reaches ∼ 10 4 K, atomic cooling turns on, and collapse ensues. We demonstrate that direct collapse proceeds in two stages, although they are not well separated. The first stage is triggered by the onset of atomic cooling, and leads to rapidly increasing accretion rate with radius, fromṀ ∼ 0.1 M ⊙ yr −1 at the halo virial radius to few M ⊙ yr −1 , around the scale radius R s ∼ 30 pc of the NFW DM density profile. The second stage of the collapse commences when the gas density takes precedence over the DM density. This is associated with the gas decoupling from the DM gravitational potential. The ensuing collapse approximates that of an isothermal sphere withṀ (r) ∼ const. We confirm that the gas loses its angular momentum through non-axisymmetric perturbations and gravitational torques, to overcome the centrifugal barrier. During the course of the collapse, this angular momentum transfer process happens on nearly all spatial scales, and the angular momentum vector of the gas varies with position and time. Collapsing gas also exhibits supersonic turbulent motions which suppress gas fragmentation, and are characterized by density PDF consisting of a lognormal part and a high-density power law tail.

We use cosmological adaptive mesh refinement (AMR) code Enzo zoom-in simulations to study the long term evolution of the collapsing gas within dark matter halos at z. This direct collapse process is a leading candidate for rapid formation of supermassive black hole (SMBH) seeds. To circumvent the Courant condition at small radii, we apply the sink particle method, focusing on evolution on scales ∼ 0.01 − 10 pc. The collapse proceeds in two stages, with the secondary runaway happening within the central 10 pc. The sink particles form when the collapsing gas requires additional refinement of the grid size at the highest refinement level. Their growth is negligible with the sole exception of the central seed which grows dramatically to M seed ∼ 2 × 10 6 M ⊙ in ∼ 2 Myr, confirming the feasibility of this path to the SMBH. The variability of angular momentum in the accreted gas results in the formation of two misaligned disks. Both disks lie within the Roche limit of the central seed. While the inner disk is geometrically thin and weakly asymmetric, the outer disk flares due to turbulent motions as a result of the massive inflow along a pair of penetrating filaments. The filamentary inflow determines the dominant Fourier modes in this disk -these modes have a non-self-gravitational origin. We do not confirm that m = 1 is a dominant mode that drives the inflow in the presence of a central massive object. The overall configuration appears to be generic, and is expected to form when the central seed becomes sufficiently massive.

We present the results of a numerical study on the effects of metal enrichment and metal cooling on galaxy formation and cosmic star formation (SF) history using cosmological hydrodynamic simulations. We find the following differences in the simulation with metal cooling when compared to the run without it: (i) the cosmic star formation rate is enhanced by about 50 and 20 per cent at z= 1 and 3, respectively; (ii) the gas mass fraction in galaxies is lower; (iii) the total baryonic mass function (gas + star) at z= 3 does not differ significantly, but shows an increase in the number of relatively massive galaxies at z= 1 and (4) the baryonic mass fraction of intergalactic medium (IGM) is reduced at z < 3 due to more efficient cooling and gas accretion on to galaxies. Our results suggest that the metal cooling enhances the galaxy growth by two different mechanisms: (i) increase in SF efficiency in the local interstellar medium and (ii) increase in IGM accretion on to galaxies. The former process is effective throughout most of the cosmic history, while the latter is effective only at z < 3 when the IGM is sufficiently enriched by metals owing to feedback.