8 research outputs found
Distribution of Forces in Gravitationally Clustered Systems
We have studied the distribution of forces in gravitational systems through
numerical experiments. Data were taken from an N-body simulation in an
expanding universe. Before clustering, the distribution of random forces was
represented as a Holtsmark distribution; the nearest-neighbor distribution is
also shown as a comparison. The analytical and simulation distributions are in
good agreement. When clustering becomes strong, the simulation result showed
that the contribution of the forces acting on each galaxy, which is generated
from all other galaxies, is almost entirely due to the gravitational attraction
of its nearest neighbor. This implies that nearest-neighbor galactic encounters
may play the main role in the dynamics of galaxy clustering.Comment: 8 pages, 4 Postscript figures, LaTeX fil
The influence of initial mass segregation on the runaway merging of stars
We have investigated the effect of initial mass segregation on the runaway
merging of stars. The evolution of multi-mass, dense star clusters was followed
by means of direct N-body simulations of up to 131.072 stars. All clusters
started from King models with dimensionless central potentials of 3.0 <= W_0 <=
9.0. Initial mass segregation was realized by varying the minimum mass of a
certain fraction of stars whose either (1) distances were closest to the
cluster center or (2) total energies were lowest. The second case is more
favorable to promote the runaway merging of stars by creating a high-mass core
of massive, low-energy stars. Initial mass segregation could decrease the
central relaxation time and thus help the formation of a high-mass core.
However, we found that initial mass segregation does not help the runaway
stellar merger to happen if the overall mass density profile is kept constant.
This is due to the fact that the collision rate of stars is not increased due
to initial mass segregation. Our simulations show that initial mass segregation
is not sufficient to allow runaway merging of stars to occur in clusters with
central densities typical for star clusters in the Milky Way.Comment: 25 pages, 9 figures, 3 tables, accepted for publication in Ap
Constraints of the Clumpyness of Dark Matter Halos Through Heating of the Disk Galaxies
Motivated by the presence of numerous dark matter clumps in the Milky
Way's halo as expected from the cold dark matter cosmological model, we
conduct numerical simulations to examine the heating of the disk. We construct
an initial galaxy model in equilibrium, with a stable thin disk.
The disk interacts with dark matter clumps for about 5 Gyr. Three physical
effects are examined : first the mass spectrum of the dark matter clumps,
second the initial thickness of the galactic disk, and third the spatial
distribution of the clumps. We find that the massive end of the mass spectrum
determines the amount of disk heating. Thicker disks suffer less heating. There
is a certain thickness at which the heating owing to the interaction with the
clumps becomes saturates. We also find that the heating produced by the model
which mimics the distribution found in
Standard CDM cosmology is significant and too high to explain the
observational constraints. On the other hand, our model that corresponds to the
clump distribution in a CDM cosmology produces no significant heating.
This result suggests that the CDM cosmology is preferable with respect
to the Standard CDM cosmology in explaining the thickness of the Milky Way.Comment: 28 pages, 13 figures, submitted to Ap
膨張宇宙中の重力的N体系における基礎的な過程
京都大学0048新制・課程博士博士(理学)甲第7937号理博第2103号新制||理||1122(附属図書館)UT51-99-M242京都大学大学院理学研究科物理学・宇宙物理学専攻(主査)教授 稲垣 省五, 教授 齋藤 衛, 助教授 石沢 俊亮学位規則第4条第1項該当Doctor of ScienceKyoto UniversityDA