171 research outputs found
Direct Collapse to Supermassive Black Hole Seeds: Comparing the AMR and SPH Approaches
We provide detailed comparison between the AMR code Enzo-2.4 and the SPH/N-
body code GADGET-3 in the context of isolated or cosmological direct baryonic
collapse within dark matter (DM) halos to form supermassive black holes. Gas
flow is examined by following evolution of basic parameters of accretion flows.
Both codes show an overall agreement in the general features of the collapse,
however, many subtle differences exist. For isolated models, the codes increase
their spatial and mass resolutions at different pace, which leads to
substantially earlier collapse in SPH than in AMR cases due to higher
gravitational resolution in GADGET-3. In cosmological runs, the AMR develops a
slightly higher baryonic resolution than SPH during halo growth via cold
accretion permeated by mergers. Still, both codes agree in the buildup of DM
and baryonic structures. However, with the onset of collapse, this difference
in mass and spatial resolution is amplified, so evolution of SPH models begins
to lag behind. Such a delay can have effect on formation/destruction rate of H2
due to UV background, and on basic properties of host halos. Finally, isolated
non-cosmological models in spinning halos, with spin parameter {\lambda} ~ 0.01
- 0.07, show delayed collapse for greater {\lambda}, but pace of this increase
is faster for AMR. Within our simulation setup, GADGET-3 requires significantly
larger computational resources than Enzo- 2.4 during collapse, and needs
similar resources, during the pre-collapse, cosmological structure formation
phase. Yet it benefits from substantially higher gravitational force and
hydrodynamic resolutions, except at the end of collapse.Comment: 19 pages, 14 figures, Referees' comments incorporated. Accepted for
publication in MNRA
DLAs and Galaxy Formation
Damped Lyman-alpha systems (DLAs) are useful probes of star formation and
galaxy formation at high redshift. We study the physical properties of DLAs and
their relationship to Lyman-break galaxies using cosmological hydrodynamic
simulations based on the concordance Lambda cold dark matter model. Fundamental
statistics such as global neutral hydrogen (HI) mass density, HI column density
distribution function, DLA rate-of-incidence and mean halo mass of DLAs are
reproduced reasonably well by the simulations, but with some deviations that
need to be understood better in the future. We discuss the feedback effects by
supernovae and galactic winds on the DLA distribution. We also compute the
[C_II] emission from neutral gas in high-z galaxies, and make predictions for
the future observations by ALMA and SPICA. Agreement and disagreement between
simulations and observations are discussed, as well as the future directions of
our DLA research.Comment: 15 pages, 10 figures. Invited brief review for Modern Physics Letters
A, in pres
Effect of radiative transfer on damped Lyman-alpha and Lyman limit systems in cosmological SPH simulations
We study the effect of local stellar radiation and UVB on the physical
properties of DLAs and LLSs at z=3 using cosmological SPH simulations. We
post-process our simulations with the ART code for radiative transfer of local
stellar radiation and UVB. We find that the DLA and LLS cross sections are
significantly reduced by the UVB, whereas the local stellar radiation does not
affect them very much except in the low-mass halos. This is because clumpy
high-density clouds near young star clusters effectively absorb most of the
ionizing photons from young stars. We also find that the UVB model with a
simple density threshold for self-shielding effect can reproduce the observed
column density distribution function of DLAs and LLSs very well, and we
validate this model by direct radiative transfer calculations of stellar
radiation and UVB with high angular resolution. We show that, with a
self-shielding treatment, the DLAs have an extended distribution around
star-forming regions typically on ~ 10-30 kpc scales, and LLSs are surrounding
DLAs on ~ 30-60 kpc scales. Our simulations suggest that the median properties
of DLA host haloes are: Mh = 2.4*10^10 Msun, SFR = 0.3 Msun/yr, M* = 2.4*10^8
Msun, and Z/Zsun = 0.1. About 30 per cent of DLAs are hosted by haloes having
SFR = 1 - 20 Msun/yr, which is the typical SFR range for LBGs. More than half
of DLAs are hosted by the LBGs that are fainter than the current observational
limit. Our results suggest that fractional contribution to LLSs from lower mass
haloes is greater than for DLAs. Therefore the median values of LLS host haloes
are somewhat lower with Mh = 9.6*10^9 Msun, SFR = 0.06 Msun/yr, M* = 6.5*10^7
Msun and Z/Zsun = 0.08. About 80 per cent of total LLS cross section are hosted
by haloes with SFR < 1 Msun/yr, hence most LLSs are associated with low-mass
halos with faint LBGs below the current detection limit.Comment: 18 pages, 12 figures, accepted for publication in MNRA
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