100 research outputs found
Hydrodynamic Simulations of Merging Galaxy Clusters: Non-Equilibrium Ionization State and Two-Temperature Structure
We investigate a non-equilibrium ionization state and an electron-ion
two-temperature structure of the intracluster medium (ICM) in merging galaxy
clusters using a series of N-body and hydrodynamic simulations. Mergers with
various sets of mass ratios and impact parameters are systematically
investigated, and it is found that, in most cases, ICM significantly departs
from the ionization equilibrium state at the shock layers with a Mach number of
~1.5-2.0 in the outskirts of the clusters, and the shock layers with a Mach
number of ~2-4 in front of the ICM cores. Accordingly, the intensity ratio
between Fe xxv and Fe xxvi K alpha line emissions is significantly altered from
that in the ionization equilibrium state. If the effect of the two-temperature
structure of ICM is incorporated, the electron temperature is ~10-20 % and
~30-50 % lower than the mean temperature of ICM at the shock layers in the
outskirts and in front of the ICM cores, respectively, and the deviation from
the ionization equilibrium state becomes larger. We also address the dependence
of the intensity ratio on the viewing angle with respect to the merging plane.Comment: 11 pages, 10 figures. Submitted to PASJ; Accepted for publication in
PAS
Direct Integration of the Collisionless Boltzmann Equation in Six-dimensional Phase Space: Self-gravitating Systems
We present a scheme for numerical simulations of collisionless
self-gravitating systems which directly integrates the Vlasov--Poisson
equations in six-dimensional phase space. By the results from a suite of
large-scale numerical simulations, we demonstrate that the present scheme can
simulate collisionless self-gravitating systems properly. The integration
scheme is based on the positive flux conservation method recently developed in
plasma physics. We test the accuracy of our code by performing several test
calculations including the stability of King spheres, the gravitational
instability and the Landau damping. We show that the mass and the energy are
accurately conserved for all the test cases we study. The results are in good
agreement with linear theory predictions and/or analytic solutions. The
distribution function keeps the property of positivity and remains
non-oscillatory. The largest simulations are run on 64^6 grids. The computation
speed scales well with the number of processors, and thus our code performs
efficiently on massively parallel supercomputers.Comment: 35 pages, 19 figures. Submitted to the Astrophysical Journa
Spatial and Dynamical Biases in Velocity Statistics of Galaxies
We present velocity statistics of galaxies and their biases inferred from the
statistics of the underlying dark matter using a cosmological hydrodynamic
simulation of galaxy formation in low-density and spatially flat cold dark
matter cosmogony. We find that the pairwise velocity dispersion (PVD) of all
galaxies is significantly lower than that of the dark matter particles, and
that the PVD of the young galaxies is lower than that of the old types, and
even of all galaxies together, especially at small separations. These results
are in reasonable agreement with the recent measurements of PVDs in the Las
Campanas redshift survey, the PSCz catalogue and the SDSS data. We also find
that the low PVD of young galaxies is due to the effects of dynamical friction
as well as the different spatial distribution. We also consider the mean infall
velocity and the POTENT density reconstruction that are often used to measure
the cosmological parameters, and investigate the effects of spatial bias and
dynamical friction. In our simulation, the mean infall velocity of young
galaxies is significantly lower than that of all the galaxies or of the old
galaxies, and the dynamical bias becomes important on scales less than 3Mpc/h.
The mass density field reconstructed from the velocity field of young galaxies
using the POTENT-style method suffers in accuracy both from the spatial bias
and the dynamical friction on the smoothing scale of R_s=8Mpc/h. On the other
hand, in the case of R_s=12Mpc/h, which is typically adopted in the actual
POTENT analysis, the density reconstruction based on various tracers of
galaxies is reasonably accurate.Comment: 29 pages, 11 figures, accepted for publication in the Ap
Non-Equilibrium Ionization State and Two-Temperature Structure in the Bullet Cluster 1E0657-56
We investigate a non-equilibrium ionization state and an electron-ion
two-temperature structure of the intracluster medium in the merging galaxy
cluster, 1E0657-56 (the Bullet cluster), using a series of N-body and
hydrodynamic simulations. We find that the electron temperature at the shock
layer associated with the X-ray sub peak (bullet) is quite different depending
on the thermal relaxation model between electrons and ions; ~25 keV for the
Coulomb thermal relaxation model and ~45 keV for the instantaneous thermal
relaxation model in the simulations which reproduce the observed X-ray
morphology. Furthermore, both of Fe xxv and Fe xxvi are overabundant compared
with the ionization equilibrium state around the shock layer, and thus, the
intensity ratio between Fe xxv and Fe xxvi K alpha lines are significantly
altered from that in the ionization equilibrium state. We also carry out the
simulations with various sets of merger parameters, and discuss a possible
range of the non-equilibrium effects in this system. Our results could be
tested with future X-ray observations such as Astro-H with better sensitivity
in high energy band.Comment: 11 pages, 9 figures. To appear in PAS
Non-Equilibrium Ionization State and Two-Temperature Structure in the Linked Region of Abell 399/401
We investigate a non-equilibrium ionization state and two-temperature
structure of the intracluster medium in the linked region of Abell 399/401,
using a series of N-body + SPH simulations, and find that there exist
significant shock layers at the edge of the linked region, and that the
ionization state of iron departs from the ionization equilibrium state at the
shock layers and around the center of the linked region. As for the
two-temperature structure, an obvious difference of temperature between
electrons and ions is found in the edge of the linked regions. K alpha line
emissions of Fe xxiv and Fe xxv are not severely affected by the deviation from
the ionization equilibrium state around the center of the linked region,
suggesting that the detection of relatively high metallicity in this area
cannot be ascribed to the non-equilibrium ionization state of the intracluster
medium. On the other hand, the K alpha emissions are significantly deviated
from the equilibrium values at the shock layers, and the intensity ratio of K
alpha lines between Fe xxiv-xxv and Fe xxvi is found to be significantly
altered from that in the ionization equilibrium state.Comment: 4 pages, 2 figures, submitted to PASJ Letter; accepted June
Non-equilibrium Ionization State of Warm-Hot Intergalactic Medium
Time evolution of the ionization state of metals in the cosmic baryons is
investigated in a cosmological context without the assumption of ionization
equilibrium. We find that a significant fraction of ionized oxygen ions (OVII
and OVIII) in the warm-hot intergalactic medium (WHIM) is not in the ionization
equilibrium state at a redshift of z~0. We also investigate the effect on the
detectability and observables of WHIM as a consequence of such deviation from
ionization equilibrium. It is found that the detectability of WHIM is not
altered very much both through its emission and absorption signatures, but line
ratios between OVII and OVIII are significantly different from those in the
ionization equilibrium state.Comment: 25 pages, 18 figures, accepted for publication in PAS
Proto-clusters in the Lambda CDM Universe
We compare the highly clustered populations of very high redshift galaxies
with proto-clusters identified numerically in a standard CDM universe
() simulation. We evolve 256^3 dark matter
particles in a comoving box of side 150h^{-1}Mpc. By the present day there are
63 cluster sized objects of mass in excess of 10^{14}h^{-1}Mo in this box. We
trace these clusters back to higher redshift finding that their progenitors at
z=4--5 are extended regions of typically 20--40 Mpc (comoving) in size, with
dark halos of mass in excess of 10^{12}h^{-1}Mo and are overdense by typically
1.3--13 times the cosmological mean density. Comparison with the observation of
Lyman alpha emitting (LAEs) galaxies at z=4.86 and at z=4.1 indicates that the
observed excess clustering is consistent with that expected for a proto-cluster
region if LAEs typically correspond to massive dark halos of more than
10^{12}h^{-1}Mo. We give a brief discussion on the relation between high
redshift concentration of massive dark halos and present day rich clusters of
galaxies.Comment: 4 pages, 5 figures, Accepted for publication in ApJ Letter
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