1,865 research outputs found
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
Supermassive Black Hole Formation at High Redshifts via Direct Collapse: Physical Processes in the Early Stage
We use numerical simulations to explore whether direct collapse can lead to
the formation of SMBH seeds at high-z. We follow the evolution of gas within DM
halos of 2 x 10^8 Mo and 1 kpc. We adopt cosmological density profiles and
j-distributions. Our goal is to understand how the collapsing flow overcomes
the centrifugal barrier and whether it is subject to fragmentation. We find
that the collapse leads either to a central runaway or to off-center
fragmentation. A disk-like configuration is formed inside the centrifugal
barrier. For more cuspy DM distribution, the gas collapses more and experiences
a bar-like perturbation and a central runaway. We have followed this inflow
down to ~10^{-4} pc. The flow remains isothermal and the specific angular
momentum is efficiently transferred by gravitational torques in a cascade of
nested bars. This cascade supports a self-similar, disk-like collapse. In the
collapsing phase, virial supersonic turbulence develops and fragmentation is
damped. For larger initial DM cores the timescales become longer. In models
with more organized initial rotation, a torus forms and appears to be supported
by turbulent motions. The evolution depends on the competition between two
timescales, corresponding to the onset of the central runaway and off-center
fragmentation. For less organized rotation, the torus is greatly weakened, the
central accretion timescale is shortened, and off-center fragmentation is
suppressed --- triggering the central runaway even in previously `stable'
models. The resulting SMBH masses lie in the range 2 x 10^4 Mo - 2 x 10^6 Mo,
much higher than for Population III remnants. We argue that the above upper
limit appears to be more realistic mass. Corollaries of this model include a
possible correlation between SMBH and DM halo masses, and similarity between
the SMBH and halo mass functions, at time of formation.Comment: 20 pages, 15 figures, 3 tables. Accepted for publication in the
Astrophysical Journa
Supermassive Black Hole Seed Formation at High Redshifts: Long-Term Evolution of the Direct Collapse
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 (DM) halos at high redshifts. This direct collapse process is a leading
candidate for rapid formation of supermassive black hole (SMBH) seeds at high
z. To circumvent the Courant condition at small radii, we have used the sink
particle method, and focus on the evolution on scales ~0.01-10 pc. The collapse
proceeds in two stages, with the secondary runaway happening within the central
10 pc, and with no detected fragmentation. The sink particles form when the
collapsing gas requires additional refinement of the grid size at the highest
refinement level. Their mass never exceeds ~10^3 Mo, with the sole exception of
the central seed which grows dramatically to ~ 2 x 10^6 Mo in ~2 Myr,
confirming the feasibility of this path to the SMBH. The time variability of
angular momentum axis in the accreted gas results in the formation of two
misaligned disks --- a small inner disk, and a more massive, outer disk which
is inclined by ~45^o to the inner disk. The self-gravity of these disks is
heavily diluted --- 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 geometry of inflow via filaments determines
the dominant and secondary Fourier modes in this disk --- these modes have a
non-self-gravitational origin. We do not confirm that m=1 is a principal mode
that drives the inflow in the presence of a central massive object. While the
positions of the disks depend on the scale chosen to break the self-similar
collapse, the overall configuration appears to be generic, and is expected to
form when the central seed becomes sufficiently massive.Comment: 14 pages, 11 figures, MNRAS, in press, typos correcte
The Baryon Cycle at High Redshifts: Effects of Galactic Winds on Galaxy Evolution in Overdense and Average Regions
We employ high-resolution cosmological zoom-in simulations focusing on a
high-sigma peak and an average cosmological field at , in order to
investigate the influence of environment and baryonic feedback on galaxy
evolution in the reionization epoch. Strong feedback, e.g., galactic winds,
caused by elevated star formation rates (SFRs) is expected to play an important
role in this evolution. We compare different outflow prescriptions: (i)
constant wind velocity (CW), (ii) variable wind scaling with galaxy properties
(VW), and (iii) no outflows (NW). The overdensity leads to accelerated
evolution of dark matter and baryonic structures, absent in the "normal"
region, and to shallow galaxy stellar mass functions at the low-mass end.
Although CW shows little dependence on both environments, the more physically
motivated VW model does exhibit this effect. In addition, VW can reproduce the
observed specific SFR (sSFR) and the sSFR-stellar mass relation, which CW and
NW fail to satisfy simultaneously. Winds also differ substantially in affecting
the state of the intergalactic medium (IGM). The difference lies in
volume-filling factor of hot, high-metallicity gas which is near unity for CW,
while it remains confined in massive filaments for VW, and locked up in
galaxies for NW. Such gas is nearly absent in the normal region. Although all
wind models suffer from deficiencies, the VW model seems to be promising in
correlating the outflow properties to those of host galaxies. Further
constraints on the state of the IGM at high- are needed to separate
different wind models.Comment: 22 pages, 15 figures, accepted for publication in the Astrophysical
Journa
Dark matter halo response to the disk growth
We consider the sensitivity of the circular-orbit adiabatic contraction
approximation to the baryon condensation rate and the orbital structure of dark
matter halos in the CDM paradigm. Using one-dimensional hydrodynamic
simulations including the dark matter halo mass accretion history and gas
cooling, we demonstrate that the adiabatic approximation is approximately valid
even though halos and disks may assemble simultaneously. We further demonstrate
the validity of the simple approximation for CDM halos with isotropic
velocity distributions using three-dimensional N-body simulations. This result
is easily understood: an isotropic velocity distribution in a cuspy halo
requires more circular orbits than radial orbits. Conversely, the approximation
is poor in the extreme case of a radial orbit halo. It overestimates the
response a core dark matter halo, where radial orbit fraction is larger.
Because no astronomically relevant models are dominated by low-angular momentum
orbits in the vicinity of the disk and the growth time scale is never shorter
than a dynamical time, we conclude that the adiabatic contraction approximation
is useful in modeling the response of dark matter halos to the growth of a
disk.Comment: 7 pages, 6 figures, accepted for publication in MNRA
Efficient Iterative V-BLAST Detection Technique in Wireless Communication System
Recently, among the MIMO-OFDM detection techniques, a lot of papers suggested V-BLAST scheme which can achieve high data rate. Therefore, the signal detection of MIMOOFDM system is important issue. In this paper, efficient iterative VBLAST detection technique is proposed in wireless communication system. The proposed scheme adjusts the number of candidate symbol and iterative scheme based on channel state. According to the simulation result, the proposed scheme has better BER performance than conventional schemes and similar BER performance of the QRD-M with iterative scheme. Moreover complexity of proposed scheme has 50.6 % less than complexity of QRD-M detection with iterative scheme. Therefore the proposed detection scheme can be efficiently used in wireless communication
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