23,835 research outputs found
Constraints on the Decomposition of the Rotation Curves of Spiral Galaxies
I discuss anew how arguments about the internal dynamics of galactic disks
set constraints on the otherwise ambiguous decomposition of the rotation curves
of spiral galaxies into the contributions by the various constituents of the
galaxies. Analyzing the two sample galaxies NGC 3198 and NGC 2985 I conclude
from the multiplicities of the spiral arms and the values of the Q disk
stability parameters that the disks of both galaxies are `maximum disks'.Comment: Latex, 6 pages, 4 figures, contributed talk at the IDM2002
conference, Sept. 2 - 6 2002, in York, Englan
Density waves in the shearing sheet III. Disc heating
The problem of dynamical heating of galactic discs by spiral density waves is
discussed using the shearing sheet model. The secular evolution of the disc is
described quantitatively by a diffusion equation for the distribution function
of stars in the space spanned by integrals of motion of the stars, in
particular the radial action integral and an integral related to the angular
momentum. Specifically, disc heating by a succession of transient, `swing
amplified' density waves is studied. It is shown that such density waves lead
predominantly to diffusion of stars in radial action space. The stochastical
changes of angular momenta of the stars and the corresponding stochastic
changes of the guiding centre radii of the stellar orbits induced by this
process are much smaller.Comment: 6 pages, 1 figure, accepted by MNRA
Jeans instability of a galactic disk embedded in a live dark halo
We investigate the Jeans instability of a galactic disk embedded in a
dynamically responsive dark halo. It is shown that the disk-halo system becomes
nominally Jeans unstable. On small scales the instability is suppressed, if the
Toomre stability index Q_T is higher than a certain threshold, but on large
scales the Jeans instability sets invariably in. However, using a simple
self-consistent disk-halo model it is demonstrated that this occurs on scales
which are much larger than the system so that this is indeed only a nominal
effect. From a practical point of view the Jeans instability of galactic disks
is not affected by a live dark halo.Comment: 3 pages, 1 figure, accepted by Astron. Astrophy
Non--Newtonian viscosity of interacting Brownian particles: comparison of theory and data
A recent first-principles approach to the non-linear rheology of dense
colloidal suspensions is evaluated and compared to simulation results of
sheared systems close to their glass transitions. The predicted scenario of a
universal transition of the structural dynamics between yielding of glasses and
non-Newtonian (shear-thinning) fluid flow appears well obeyed, and calculations
within simplified models rationalize the data over variations in shear rate and
viscosity of up to 3 decades.Comment: 6 pages, 2 figures; J. Phys. Condens. Matter to be published (Jan.
2003
- …