86 research outputs found
FAST: A Fully Asynchronous Split Time-Integrator for Self-Gravitating Fluid
We describe a new algorithm for the integration of self-gravitating fluid
systems using SPH method. We split the Hamiltonian of a self-gravitating fluid
system to the gravitational potential and others (kinetic and internal
energies) and use different time-steps for their integrations. The time
integration is done in the way similar to that used in the mixed variable or
multiple stepsize symplectic schemes. We performed three test calculations. One
was the spherical collapse and the other was an explosion. We also performed a
realistic test, in which the initial model was taken from a simulation of
merging galaxies. In all test calculations, we found that the number of
time-steps for gravitational interaction were reduced by nearly an order of
magnitude when we adopted our integration method. In the case of the realistic
test, in which the dark matter potential dominates the total system, the total
calculation time was significantly reduced. Simulation results were almost the
same with those of simulations with the ordinary individual time-step method.
Our new method achieves good performance without sacrificing the accuracy of
the time integration.Comment: 14 pages, 8 figures, accepted for publication in PAS
Properties of thick discs formed in clumpy galaxies
We examine a possible formation scenario of galactic thick discs with
numerical simulations. Thick discs have previously been argued to form in
clumpy disc phase in the high-redshift Universe, which host giant clumps of
<10^9 M_sun in their highly gas-rich discs. We performed SPH simulations using
isolated galaxy models for the purpose of verifying whether dynamical and
chemical properties of the thick discs formed in such clumpy galaxies are
compatible with observations. The results of our simulations seem nearly
consistent with observations in dynamical properties such as radial and
vertical density profiles, significant rotation velocity lag with height and
distributions of orbital eccentricities. In addition, the thick discs in our
simulations indicate nearly exponential dependence of azimuthal and vertical
velocity dispersions with radius, nearly isothermal kinematics in vertical
direction and negligible metallicity gradients in radial and vertical
directions. However, our simulations cannot reproduce altitudinal dependence of
eccentricities, metallicity relations with eccentricities or rotation
velocities, which shows striking discrepancy from recent observations of the
Galactic thick disc. From this result, we infer that the clumpy disc scenario
for thick-disc formation would not be suitable at least for the Milky Way. Our
study, however, cannot reject this scenario for external galaxies if not all
galaxies form their thick discs by the same process. In addition, we found that
a large fraction of thick-disc stars forms in giant clumps.Comment: 15 pages, 13 figures, 3 tables, accepted for publication in MNRA
On the Interpretation of the l-v Features in the Milky Way Galaxy
We model the gas dynamics of barred galaxies using a three-dimensional,
high-resolution, -body+hydrodynamical simulation and apply it to the Milky
Way in an attempt to reproduce both the large-scale structure and the clumpy
morphology observed in Galactic H\emissiontype{I} and CO diagrams. Owing
to including the multi-phase interstellar medium, self-gravity, star-formation
and supernovae feedback, the clumpy morphology, as well as the large-scale
features, in observed diagrams are naturally reproduced. We identify in
our diagrams with a number of not only large-scale peculiar features such
as the '3-kpc arm', '135-km s arm' and 'Connecting arm' but also clumpy
features such as `Bania clumps', and then link these features in a face-on view
of our model. We give suggestions on the real structure of the Milky Way and on
the fate of gas clumps in the central region.Comment: accepted to PAS
The Giant Impact Simulations with Density Independent Smoothed Particle Hydrodynamics
At present, the giant impact (GI) is the most widely accepted model for the
origin of the Moon. Most of the numerical simulations of GI have been carried
out with the smoothed particle hydrodynamics (SPH) method. Recently, however,
it has been pointed out that standard formulation of SPH (SSPH) has
difficulties in the treatment of a contact discontinuity such as a core-mantle
boundary and a free surface such as a planetary surface. This difficulty comes
from the assumption of differentiability of density in SSPH. We have developed
an alternative formulation of SPH, density independent SPH (DISPH), which is
based on differentiability of pressure instead of density to solve the problem
of a contact discontinuity. In this paper, we report the results of the GI
simulations with DISPH and compare them with those obtained with SSPH. We found
that the disk properties, such as mass and angular momentum produced by DISPH
is different from that of SSPH. In general, the disks formed by DISPH are more
compact: while formation of a smaller mass moon for low-oblique impacts is
expected with DISPH, inhibition of ejection would promote formation of a larger
mass moon for high-oblique impacts. Since only the improvement of core-mantle
boundary significantly affects the properties of circumplanetary disks
generated by GI and DISPH has not been significantly improved from SSPH for a
free surface, we should be very careful when some conclusions are drawn from
the numerical simulations for GI. And it is necessary to develop the numerical
hydrodynamical scheme for GI that can properly treat the free surface as well
as the contact discontinuity.Comment: Accepted for publication in Icaru
- …