565 research outputs found
Determining the galactic mass distribution using tidal streams from globular clusters
We discuss how to use tidal streams from globular clusters to measure the
mass distribution of the Milky Way. Recent proper motion determinations for
globular clusters from plate measurements and Hipparcos astrometry provide
several good candidates for Galactic mass determinations in the intermediate
halo, far above the Galactic disk, including Pal 5, NGC 4147, NGC 5024 (M53)
and NGC 5466; the remaining Hipparcos clusters provide candidates for
measurements several kpc above and below the disk. These clusters will help
determine the profile and shape of the inner halo. To aid this effort, we
present two methods of mass determination: one, a generalization of
rotation-curve mass measurements, which gives the mass and potential from
complete position-velocity observations for stream stars; and another using a
simple chi^2 estimator, which can be used when only projected positions and
radial velocities are known for stream stars. We illustrate the use of the
latter method using simulated tidal streams from Pal 5 and find that fairly
accurate mass determinations are possible even for relatively poor data sets.
Follow-up observations of clusters with proper motion determinations may reveal
tidal streams; obtaining radial velocity measurements would enable accurate
measurements of the mass distribution in the inner Galaxy.Comment: 21 pages, 6 figures, published in A
Intrinsic Shapes of Molecular Cloud Cores
We conduct an analysis of the shapes of molecular cloud cores using recently
compiled catalogs of observed axis ratios of individual cores mapped in ammonia
or through optical selection. We apply both analytical and statistical
techniques to deproject the observed axis ratios in order to determine the true
distribution of cloud core shapes. We find that neither pure oblate nor pure
prolate cores can account for the observed distribution of core shapes.
Intrinsically triaxial cores produce distributions which agree with
observations. The best-fit triaxial distribution contains cores which are more
nearly oblate than prolate.Comment: 20 pages, 10 figures. To appear in ApJ (2001 April 1). Color figures
available at http://www.astro.uwo.ca/~cjones/ or
http://www.astro.uwo.ca/~basu/pub.htm
Substructure around M31 : Evolution and Effects
We investigate the evolution of a population of 100 dark matter satellites
orbiting in the gravitational potential of a realistic model of M31. We find
that after 10 Gyr, seven subhalos are completely disrupted by the tidal field
of the host galaxy. The remaining satellites suffer heavy mass loss and
overall, 75% of the mass initially in the subhalo system is tidally stripped.
Not surprisingly, satellites with pericentric radius less than 30 kpc suffer
the greatest stripping and leave a complex structure of tails and streams of
debris around the host galaxy. Assuming that the most bound particles in each
subhalo are kinematic tracers of stars, we find that the halo stellar
population resulting from the tidal debris follows an r^{-3.5} density profile
at large radii. We construct B-band photometric maps of stars coming from
disrupted satellites and find conspicuous features similar both in morphology
and brightness to the observed Giant Stream around Andromeda. An assumed star
formation efficiency of 5-10% in the simulated satellite galaxies results in
good agreement with the number of M31 satellites, the V-band surface brightness
distribution, and the brightness of the Giant Stream. During the first 5 Gyr,
the bombardment of the satellites heats and thickens the disk by a small
amount. At about 5 Gyr, satellite interations induce the formation of a strong
bar which, in turn, leads to a significant increase in the velocity dispersion
of the disk.Comment: 45 pages, 18 figures. To be submitted to the Astrophysical Journal,
version 2.0 : scale height value corrected, references added, and some
figures have been modifie
The Origin of the Brightest Cluster Galaxies
Most clusters and groups of galaxies contain a giant elliptical galaxy in
their centres which far outshines and outweighs normal ellipticals. The origin
of these brightest cluster galaxies is intimately related to the collapse and
formation of the cluster. Using an N-body simulation of a cluster of galaxies
in a hierarchical cosmological model, we show that galaxy merging naturally
produces a massive, central galaxy with surface brightness and velocity
dispersion profiles similar to observed BCG's. To enhance the resolution of the
simulation, 100 dark halos at are replaced with self-consistent
disk+bulge+halo galaxy models following a Tully-Fisher relation using 100000
particles for the 20 largest galaxies and 10000 particles for the remaining
ones. This technique allows us to analyze the stellar and dark matter
components independently. The central galaxy forms through the merger of
several massive galaxies along a filament early in the cluster's history.
Galactic cannibalism of smaller galaxies through dynamical friction over a
Hubble time only accounts for a small fraction of the accreted mass. The galaxy
is a flattened, triaxial object whose long axis aligns with the primordial
filament and the long axis of the cluster galaxy distribution agreeing with
observed trends for galaxy-cluster alignment.Comment: Revised and accepted in ApJ, 25 pages, 10 figures, online version
available at http://www.cita.utoronto.ca/~dubinski/bcg
Stellar Disks in Aquarius Dark Matter Haloes
We investigate the gravitational interactions between live stellar disks and
their dark matter halos, using LCDM haloes similar in mass to that of the Milky
Way taken from the Aquarius Project. We introduce the stellar disks by first
allowing the haloes to respond to the influence of a growing rigid disk
potential from z = 1.3 to z = 1.0. The rigid potential is then replaced with
star particles which evolve self-consistently with the dark matter particles
until z = 0.0. Regardless of the initial orientation of the disk, the inner
parts of the haloes contract and change from prolate to oblate as the disk
grows to its full size. When the disk normal is initially aligned with the
major axis of the halo at z=1.3, the length of the major axis contracts and
becomes the minor axis by z=1.0. Six out of the eight disks in our main set of
simulations form bars, and five of the six bars experience a buckling
instability that results in a sudden jump in the vertical stellar velocity
dispersion and an accompanying drop in the m=2 Fourier amplitude of the disk
surface density. The bars are not destroyed by the buckling but continue to
grow until the present day. Bars are largely absent when the disk mass is
reduced by a factor of two or more; the relative disk-to-halo mass is therefore
a primary factor in bar formation and evolution. A subset of the disks is
warped at the outskirts and contains prominent non-coplanar material with a
ring-like structure. Many disks reorient by large angles between z=1 and z=0,
following a coherent reorientation of their inner haloes. Larger reorientations
produce more strongly warped disks, suggesting a tight link between the two
phenomena. The origins of bars and warps appear independent: some disks with
strong bars show no disturbances at the outskirts, while the disks with the
weakest bars show severe warps.Comment: 19 pages, 13 figures, accepted MNRAS; fixed compatibility problem in
figures 8,
Periodic orbits in warped disk
It is often assumed that a warped galaxy can be modeled by a set of rings.
This paper verifies numerically the validity of this assumption by the study of
periodic orbits populating a heavy self-gravitating warped disk. The phase
space structure of a warped model reveals that the circular periodic orbits of
a flat disk are transformed in quasi annular periodic orbits which conserve
their stability. This lets us also explore the problem of the persistence of a
large outer warp. In particular, the consistency of its orbits with the density
distribution is checked as a function of the pattern speed.Comment: 9 pages, including 11 figures. Accepted for publication in A&
Searching for Machos (and other Dark Matter Candidates) in a Simulated Galaxy
We conduct gravitational microlensing experiments in a galaxy taken from a
cosmological N-body simulation. Hypothetical observers measure the optical
depth and event rate toward hypothetical LMCs and compare their results with
model predictions. Since we control the accuracy and sophistication of the
model, we can determine how good it has to be for statistical errors to
dominate over systematic ones. Several thousand independent microlensing
experiments are performed. When the ``best-fit'' triaxial model for the mass
distribution of the halo is used, the agreement between the measured and
predicted optical depths is quite good: by and large the discrepancies are
consistent with statistical fluctuations. If, on the other hand, a spherical
model is used, systematic errors dominate. Even with our ``best-fit'' model,
there are a few rare experiments where the deviation between the measured and
predicted optical depths cannot be understood in terms of statistical
fluctuations. In these experiments there is typically a clump of particles
crossing the line of sight to the hypothetical LMC. These clumps can be either
gravitationally bound systems or transient phenomena in a galaxy that is still
undergoing phase mixing. Substructure of this type, if present in the Galactic
distribution of Machos, can lead to large systematic errors in the analysis of
microlensing experiments. We also describe how hypothetical WIMP and axion
detection experiments might be conducted in a simulated N-body galaxy.Comment: 18 pages of text (LaTeX, AASTeX) with 12 figures. submitted to the
Astrophysical Journa
Parallel TREE code for two-component ultracold plasma analysis
The TREE method has been widely used for long-range interaction {\it N}-body
problems. We have developed a parallel TREE code for two-component classical
plasmas with open boundary conditions and highly non-uniform charge
distributions. The program efficiently handles millions of particles evolved
over long relaxation times requiring millions of time steps. Appropriate domain
decomposition and dynamic data management were employed, and large-scale
parallel processing was achieved using an intermediate level of granularity of
domain decomposition and ghost TREE communication. Even though the
computational load is not fully distributed in fine grains, high parallel
efficiency was achieved for ultracold plasma systems of charged particles. As
an application, we performed simulations of an ultracold neutral plasma with a
half million particles and a half million time steps. For the long temporal
trajectories of relaxation between heavy ions and light electrons, large
configurations of ultracold plasmas can now be investigated, which was not
possible in past studies
Dark matter response to galaxy formation
We have resimulated the six galaxy-sized haloes of the Aquarius Project
including metal-dependent cooling, star formation and supernova feedback. This
allows us to study not only how dark matter haloes respond to galaxy formation,
but also how this response is affected by details of halo assembly history. In
agreement with previous work, we find baryon condensation to lead to increased
dark matter concentration. Dark matter density profiles differ substantially in
shape from halo to halo when baryons are included, but in all cases the
velocity dispersion decreases monotonically with radius. Some haloes show an
approximately constant dark matter velocity anisotropy with , while others retain the anisotropy structure of their baryon-free
versions. Most of our haloes become approximately oblate in their inner
regions, although a few retain the shape of their dissipationless counterparts.
Pseudo-phase-space densities are described by a power law in radius of altered
slope when baryons are included. The shape and concentration of the dark matter
density profiles are not well reproduced by published adiabatic contraction
models. The significant spread we find in the density and kinematic structure
of our haloes appears related to differences in their formation histories. Such
differences already affect the final structure in baryon-free simulations, but
they are reinforced by the inclusion of baryons, and new features are produced.
The details of galaxy formation need to be better understood before the inner
dark matter structure of galaxies can be used to constrain cosmological models
or the nature of dark matter.Comment: 14 pages, 9 figures. Accepted MNRAS. Revised version includes
discussion on resolution effects and minor changes
XMMU J100750.5+125818: A strong lensing cluster at z=1.082
We report on the discovery of the X-ray luminous cluster XMMU
J100750.5+125818 at redshift 1.082 based on 19 spectroscopic members, which
displays several strong lensing features. SED modeling of the lensed arc
features from multicolor imaging with the VLT and the LBT reveals likely
redshifts ~2.7 for the most prominent of the lensed background galaxies. Mass
estimates are derived for different radii from the velocity dispersion of the
cluster members, M_200 ~ 1.8 10^{14} Msun, from the X-ray spectral parameters,
M_500 ~ 1.0 10^{14} Msun, and the largest lensing arc, M_SL ~ 2.3 10^{13} Msun.
The projected spatial distribution of cluster galaxies appears to be elongated,
and the brightest galaxy lies off center with respect to the X-ray emission
indicating a not yet relaxed structure. XMMU J100750.5+125818 offers excellent
diagnostics of the inner mass distribution of a distant cluster with a
combination of strong and weak lensing, optical and X-ray spectroscopy.Comment: A&A, accepted for publicatio
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