707 research outputs found
On the Destruction and Over-Merging of Dark Halos in Dissipationless N-body Simulations
N-body simulations that follow only a collisionless dark matter component
have failed to produce galaxy halos or substructure within dense environments.
We investigate the `over-merging' problem analytically and with numerical
simulations, by calculating dissolution timescales of halos due to physical and
artificial dynamical effects. The numerical resolution that has recently been
attained is such that mass-loss from two-body relaxation is negligible. We
demonstrate that substructure is destroyed in present simulations as a result
of large force softening combined with the heating sources of tides and
encounters with dissolving substructure. In the limit of infinite numerical
resolution, whether or not individual halos or substructure can survive depends
sensitively on their inner density profiles. Singular isothermal halos will
always survive at some level, however, if halos form with large core radii then
the over-merging problem will always exist within dissipationless N-body
simulations. In this latter case a dissipational component can increase the
halos central density enabling galaxies to survive.Comment: submitted to ApJL. compressed postscript file includes figures
Formation of Proto-Globular Cluster Clouds by Thermal Instability
Many models of globular cluster formation assume the presence of cold dense
clouds in early universe. Here we re-examine the Fall & Rees (1985) model for
formation of proto-globular cluster clouds (PGCCs) via thermal instabilities in
a protogalactic halo. We first argue, based on the previous study by others,
that under the protogalactic environments only nonlinear density
inhomogeneities can condense into PGCCs. We then carry out numerical
simulations of the collapse of overdense clouds in one-dimensional spherical
geometry, including self-gravity and radiative cooling down to T=10^4 K. Since
imprinting of Jeans mass at 10^4 K is essential to this model, here we focus on
the cases where external UV background radiation prevents the formation of H2
molecules and so prevent the cloud from cooling below 10^4 K. The quantitative
results from these simulations can be summarized as follows: 1) Perturbations
smaller than M_min ~ (10^{5.6} M_sun) (n_h/0.05 cm3)^{-2} cool isobarically,
while perturbations larger than M_max ~ (10^8 M_sun) (n_h/0.05 cm3)^{-2} cool
isochorically. On the other hand, intermediate size perturbations (M_min<
M_pgcc < M_max) are compressed supersonically. 2) For supersonically collapsing
clouds, the density compression factor after they cool to T_c=10^4 K range
10^{2.5}-10^6. 3) For supersonically collapsing clouds the Jeans mass can be
reduced to as small as 10^{5.5} M_sun (n_h/0.05 cm3)^{-1/2} at the maximum
compression. 4) The density profile of simulated PGCCs can be approximated by a
constant core with a halo of rho ~ r^{-2} rather than a singular isothermal
sphere.Comment: 11 pages, to appear in Journal of Korean Astronomical Society, uses
jkas2.st
Measuring the local dark matter density
We examine systematic problems in determining the local matter density from
the vertical motion of stars, i.e. the 'Oort limit'. Using collisionless
simulations and a Monte Carlo Markov Chain technique, we determine the data
quality required to detect local dark matter at its expected density. We find
that systematic errors are more important than observational errors and apply
our technique to Hipparcos data to reassign realistic error bars to the local
dark matter density.Comment: 3 pages, 1 figure, to be published in "Hunting for the Dark: The
Hidden Side of Galaxy Formation", Malta, 19-23 Oct. 2009, eds. V.P.
Debattista & C.C. Popescu, AIP Conf. Se
The Magellanic Group and the Seven Dwarfs
The Magellanic Clouds were the largest members of a group of dwarf galaxies
that entered the Milky Way (MW) halo at late times. This group, dominated by
the LMC, contained ~4% of the mass of the Milky Way prior to its accretion and
tidal disruption, but ~70% of the known dwarfs orbiting the MW. Our theory
addresses many outstanding problems in galaxy formation associated with dwarf
galaxies. First, it can explain the planar orbital configuration populated by
some dSphs in the MW. Second, it provides a mechanism for lighting up a subset
of dwarf galaxies to reproduce the cumulative circular velocity distribution of
the satellites in the MW. Finally, our model predicts that most dwarfs will be
found in association with other dwarfs. The recent discovery of Leo V
(Belokurov et al. 2008), a dwarf spheroidal companion of Leo IV, and the nearby
dwarf associations supports our hypothesis.Comment: Contributed talk to IAU Symposium 256: "The Magellanic System: Stars,
Gas, and Galaxies
The Magellanic Group and the Seven Dwarfs
The Magellanic Clouds were the largest members of a group of dwarf galaxies that entered the Milky Way (MW) halo at late times. This group, dominated by the LMC, contained ~4% of the mass of the Milky Way prior to its accretion and tidal disruption, but ≈70% of the known dwarfs orbiting the MW. Our theory addresses many outstanding problems in galaxy formation associated with dwarf galaxies. First, it can explain the planar orbital configuration populated by some dSphs in the MW. Second, it provides a mechanism for lighting up a subset of dwarf galaxies to reproduce the cumulative circular velocity distribution of the satellites in the MW. Finally, our model predicts that most dwarfs will be found in association with other dwarfs. The recent discovery of Leo V (Belokurov et al. 2008), a dwarf spheroidal companion of Leo IV, and the nearby dwarf associations supports our hypothesi
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