448 research outputs found
Stellar Velocity Distribution in Galactic Disks
We present numerical studies of the properties of the stellar velocity
distribution in galactic disks which have developed a saturated, two-armed
spiral structure. In previous papers we used the Boltzmann moment equations
(BME) up to second order for our studies of the velocity structure in
self-gravitating stellar disks. A key assumption of our BME approach is the
zero-heat flux approximation, i.e. the neglection of third order velocity
terms. We tested this assumption by performing test particle simulations for
stars in a disk galaxy subject to a rotating spiral perturbation. As a result
we corroborated qualitatively the complex velocity structure found in the BME
approach. It turned out that an equilibrium configuration in velocity space is
only slowly established on a typical timescale of 5 Gyrs or more. Since many
dynamical processes in galaxies (like the growth of spirals or bars)act on
shorter timescales, pure equilibrium models might not be fully appropriate for
a detailed comparison with observations like the local Galactic velocity
distribution. Third order velocity moments were typically small and
uncorrelated over almost all of the disk with the exception of the 4:1
resonance region (UHR). Near the UHR (normalized) fourth and fifth order
velocity moments are still of the same order as the second and third order
terms. Thus, at the UHR higher order terms are not negligible.Comment: 8 pages, 3 figures, to appear in the Proceedings of "Chaos in
Astronomy", Athens, G. Contopoulos & P.A. Patsis (eds.
Spatial motion of the Magellanic Clouds. Tidal models ruled out?
Recently, Kallivayalil et al. derived new values of the proper motion for the
Large and Small Magellanic Clouds (LMC and SMC, respectively). The spatial
velocities of both Clouds are unexpectedly higher than their previous values
resulting from agreement between the available theoretical models of the
Magellanic System and the observations of neutral hydrogen (HI) associated with
the LMC and the SMC. Such proper motion estimates are likely to be at odds with
the scenarios for creation of the large-scale structures in the Magellanic
System suggested so far. We investigated this hypothesis for the pure tidal
models, as they were the first ones devised to explain the evolution of the
Magellanic System, and the tidal stripping is intrinsically involved in every
model assuming the gravitational interaction. The parameter space for the Milky
Way (MW)-LMC-SMC interaction was analyzed by a robust search algorithm (genetic
algorithm) combined with a fast restricted N-body model of the interaction. Our
method extended the known variety of evolutionary scenarios satisfying the
observed kinematics and morphology of the Magellanic large-scale structures.
Nevertheless, assuming the tidal interaction, no satisfactory reproduction of
the HI data available for the Magellanic Clouds was achieved with the new
proper motions. We conclude that for the proper motion data by Kallivayalil et
al., within their 1-sigma errors, the dynamical evolution of the Magellanic
System with the currently accepted total mass of the MW cannot be explained in
the framework of pure tidal models. The optimal value for the western component
of the LMC proper motion was found to be pm_w(LMC) > -1.3 mas/yr in case of
tidal models. It corresponds to the reduction of the Kallivayalil et al. value
for pm_w(LMC) by approx. 40% in its magnitude.Comment: ApJ accepted, 17 pages, 4 figure
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