888 research outputs found
One-Armed Spiral Waves in Galaxy Simulations with Counter-Rotating Stars
Motivated by observations of disk galaxies with counter-rotating stars, we
have run two-dimensional, collisionless N-body simulations of disk galaxies
with significant counter-rotating components. For all our simulations the
initial value of Toomre's stability parameter was Q = 1.1. The percentage of
counter-rotating particles ranges from 25% to 50%. A stationary one-arm spiral
wave is observed to form in each run, persisting from a few to five rotation
periods, measured at the half-mass radius. In one run, the spiral wave was
initially a leading arm which subsequently transformed into a trailing arm. We
also observed a change in spiral direction in the run initially containing
equal numbers of particles orbiting in both directions. The results of our
simulations support an interpretation of the one armed waves as due to the two
stream instability.Comment: 13 pages, 4 figure
Growth of Velocity Dispersions for Collapsing Spherical Stellar Systems
First, we have ensured that spherical nonrotating collisionless systems
collapse with almost retaining spherical configurations during initial
contraction phases even if they are allowed to collapse three-dimensionally.
Next, on the assumption of spherical symmetry, we examine the evolution of
velocity dispersions with collapse for the systems which have uniform or
power-law density profiles with Maxwellian velocity distributions by
integrating the collisionless Boltzmann equation directly. The results show
that as far as the initial contraction phases are concerned, the radial
velocity dispersion never grows faster than the tangential velocity dispersion
except at small radii where the nearly isothermal nature remains, irrespective
of the density profiles and virial ratios. This implies that velocity
anisotropy as an initial condition should be a poor indicator for the radial
orbit instability. The growing behavior of the velocity dispersions is briefly
discussed from the viewpoint that phase space density is conserved in
collisionless systems.Comment: 12 pages, including 5 postscript figures. This preprint is also
available at http://www.kcua.ac.jp/~fujiwara/e-prints/e-prints.html Submitted
to Publ.Astron.Soc.Japa
A Long Slit-Like Entrance Promotes Ventilation in the Mud Nesting Social Wasp, Polybia spinifex: Visualization of Nest Microclimates using Computational Fluid Dynamics
Polybia spinifex Richards (Hymenoptera: Vespidae) constructs mud nests characterized by a long slit-like entrance. The ventilation and thermal characteristics of the P. spinifex nest were investigated to determine whether the nest microclimate is automatically maintained due to the size of the entrance. In order to examine this hypothesis, a numerical simulation was employed to predict the effects of the entrance length. The calculations were performed with 3D-virtual models that simulated the P. spinifex nest conditions, and the reliability of the simulations was experimentally examined by using gypsum-model nests and a P. spinifex nest. The ventilation effect was determined by blowing air through the nest at 1â3 m/s (airflow conditions); the airspeed was found to be higher in models with a longer entrance. The ventilation rate was also higher in models with longer entrances, suggesting that the P. spinifex nest is automatically ventilated by natural winds. Next, the thermal effect was calculated under condition of direct sunlight. Under a calm condition (airflow, 0 m/s), thermal convection and a small temperature drop were observed in the case of models with a long entrance, whereas the ventilation and thermoregulation effects seemed small. Under airflow conditions, the temperature at the mid combs steeply dropped due to the convective airflow through the entrance at 1â2 m/s, and at 3 m/s, most of the heat was eliminated due to high thermal conductivity of the mud envelope, rather than convection
A Phase-Space Approach to Collisionless Stellar Systems Using a Particle Method
A particle method for reproducing the phase space of collisionless stellar
systems is described. The key idea originates in Liouville's theorem which
states that the distribution function (DF) at time t can be derived from
tracing necessary orbits back to t=0. To make this procedure feasible, a
self-consistent field (SCF) method for solving Poisson's equation is adopted to
compute the orbits of arbitrary stars. As an example, for the violent
relaxation of a uniform-density sphere, the phase-space evolution which the
current method generates is compared to that obtained with a phase-space method
for integrating the collisionless Boltzmann equation, on the assumption of
spherical symmetry. Then, excellent agreement is found between the two methods
if an optimal basis set for the SCF technique is chosen. Since this
reproduction method requires only the functional form of initial DFs but needs
no assumptions about symmetry of the system, the success in reproducing the
phase-space evolution implies that there would be no need of directly solving
the collisionless Boltzmann equation in order to access phase space even for
systems without any special symmetries. The effects of basis sets used in SCF
simulations on the reproduced phase space are also discussed.Comment: 16 pages w/4 embedded PS figures. Uses aaspp4.sty (AASLaTeX v4.0). To
be published in ApJ, Oct. 1, 1997. This preprint is also available at
http://www.sue.shiga-u.ac.jp/WWW/prof/hozumi/papers.htm
Evolution of Massive Blackhole Triples I -- Equal-mass binary-single systems
We present the result of -body simulations of dynamical evolution of
triple massive blackhole (BH) systems in galactic nuclei. We found that in most
cases two of the three BHs merge through gravitational wave (GW) radiation in
the timescale much shorter than the Hubble time, before ejecting one BH through
a slingshot. In order for a binary BH to merge before ejecting out the third
one, it has to become highly eccentric since the gravitational wave timescale
would be much longer than the Hubble time unless the eccentricity is very high.
We found that two mechanisms drive the increase of the eccentricity of the
binary. One is the strong binary-single BH interaction resulting in the
thermalization of the eccentricity. The second is the Kozai mechanism which
drives the cyclic change of the inclination and eccentricity of the inner
binary of a stable hierarchical triple system. Our result implies that many of
supermassive blackholes are binaries.Comment: 20 pages, 12 figure
The Self-Regulated Growth of Supermassive Black Holes
We present a series of simulations of the self--regulated growth of
supermassive black holes (SMBHs) in galaxies via three different fueling
mechanisms: major mergers, minor mergers, and disk instabilities. The SMBHs in
all three scenarios follow the same black hole fundamental plane (BHFP) and
correlation with bulge binding energy seen in simulations of major mergers, and
observed locally. Furthermore, provided that the total gas supply is
significantly larger than the mass of the SMBH, its limiting mass is not
influenced by the amount of gas available or the efficiency of black hole
growth. This supports the assertion that SMBHs accrete until they reach a
critical mass at which feedback is sufficient to unbind the gas locally,
terminating the inflow and stalling further growth. At the same time, while
minor and major mergers follow the same projected correlations (e.g., the
and Magorrian relations), SMBHs grown via disk instabilities do
not, owing to structural differences between the host bulges. This finding is
supported by recent observations of SMBHs in pseudobulges and bulges in barred
systems, as compared to those hosted by classical bulges. Taken together, this
provides support for the BHFP and binding energy correlations as being more
"fundamental" than other proposed correlations in that they reflect the
physical mechanism driving the co-evolution of SMBHs and spheroids.Comment: 15 pages, 16 figures, accepted for publication in Ap
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