Jun-Hwan Choi

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 = 10 9 -10 12 M ⊙ . 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 ∼ 10 9 M ⊙ have large values of f esc (with an average of ∼0.4), whereas the massive haloes with M h ∼ 10 11 M ⊙ 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 ≲ 10 10 M ⊙ owing to their high f esc . The large dispersion in f esc 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 f esc using simple, four different treatments of UV background.

The dynamics of tidal tails from massive satellites

We investigate the dynamical mechanisms responsible for producing tidal tails from dwarf satellites using N-body simulations. We describe the essential dynamical mechanisms and morphological consequences of tail production in satellites with masses greater than 0.0001 of the host halo virial mass. We identify two important dynamical co-conspirators: 1) the points where the attractive force of the host halo and satellite are balanced (X-points) do not occur at equal distances from the satellite centre or at the same equipotential value for massive satellites, breaking the morphological symmetry of the leading and trailing tails; and 2) the escaped ejecta in the leading (trailing) tail continues to be decelerated (accelerated) by the satellite’s gravity leading to large offsets of the ejecta orbits from the satellite orbit. The effect of the satellite’s self gravity decreases only weakly with a decreasing ratio of satellite mass to host halo mass, proportional to (Ms/Mh) 1/3 , demonstrating the importance of these effects over a wide range of subhalo masses. Not only will the morphology of the leading and trailing tails for massive satellites be different, but the observed radial velocities of the tails will be displaced from that of the satellite orbit; both the displacement and the maximum radial velocity is proportional to satellite mass. If the tails are assumed to follow the progenitor satellite orbits, the tails from satellites with masses greater than 0.0001 of the host halo virial mass in a spherical halo will appear to indicate a flattened halo. Therefore, a constraint on the Milky Way halo shape using tidal streams requires mass-dependent modelling. Similarly, we compute the the distribution of tail orbits both in Er r 2 space (Lynden-Bell & Lynden-Bell 1995) and in E Lz space (Helmi & de Zeeuw 2000), advocated for identifying satellite stream relics. The acceleration of ejecta by a massive satellite during escape spreads the velocity distribution and obscures the signature of a well-defined “moving group” in phase space. Although these findings complicate the interpretation of stellar streams and moving groups, the intrinsic mass dependence provides additional leverage on both halo and progenitor satellite properties.

MOLECULAR HYDROGEN REGULATED STAR FORMATION IN COSMOLOGICAL SMOOTHED PARTICLE HYDRODYNAMICS SIMULATIONS

Some observations have shown that star formation (SF) correlates tightly with the presence of molecular hydrogen (H2); therefore, it is important to investigate its implication on galaxy formation in a cosmological context. In the present work, we implement a sub-grid model (hereafter H2-SF model) that tracks the H2 mass fraction within our cosmological smoothed particle hydrodynamics code GADGET-3 by using an equilibrium analytic model of Krumholz et al. This model allows us to regulate the SF in our simulation by the local abundance of H2 rather than the total cold gas density, which naturally introduces the dependence of SF on metallicity. We investigate the implications of the H2-SF model on galaxy population properties, such as the stellar-to-halo mass ratio (SHMR), baryon fraction, cosmic star formation rate density (SFRD), galaxy specific SFR, galaxy stellar mass functions (GSMF), and Kennicutt-Schmidt (KS) relationship. The advantage of our work over the previous ones is having a large sample of simulated galaxies in a cosmological volume from high redshift to z = 0. We find that low-mass halos with M DM < 1010.5 M ☉ are less efficient in producing stars in the H2-SF model at z ≥ 6, which brings the simulations into better agreement with the observational estimates of the SHMR and GSMF at the low-mass end. This is particularly evident by a reduction in the number of low-mass galaxies at M ≤ 108 M ☉ in the GSMF. The overall SFRD is also reduced at high z in the H2 run, which results in slightly higher SFRD at low redshift due to more abundant gas available for SF at later times. This new H2 model is able to reproduce the empirical KS relationship at z = 0 naturally, without the need for setting its normalization by hand, and overall it seems to have more advantages than the previous pressure-based SF model.

Supermassive Black Hole Formation at High Redshifts via Direct Collapse in a Cosmological Context

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 high-resolution zoom-in simulations of such collapse at high-

Duty cycle and the increasing star formation history of z ≥ 6 galaxies

z

Effects of cosmological parameters and star formation models on the cosmic star formation history in ΛCDM cosmological simulations

. Using the adaptive mesh refinement code ENZO, we resolve the formation and growth of a DM halo, until its virial temperature reaches

Effect of radiative transfer on damped Lyα and Lyman limit systems in cosmological SPH simulations

\sim 10^4

Effects of metal enrichment and metal cooling in galaxy growth and cosmic star formation history

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, from

On the inconsistency between the estimates of cosmic star formation rate and stellar mass density of high-redshift galaxies

\dot M\sim 0.1\,M_\odot {\rm yr^{-1}}

The dynamics of satellite disruption in cold dark matter haloes

at the halo virial radius to few