346 research outputs found
Identikit 2: An Algorithm for Reconstructing Galactic Collisions
Using a combination of self-consistent and test-particle techniques,
Identikit 1 provided a way to vary the initial geometry of a galactic collision
and instantly visualize the outcome. Identikit 2 uses the same techniques to
define a mapping from the current morphology and kinematics of a tidal
encounter back to the initial conditions. By requiring that various regions
along a tidal feature all originate from a single disc with a unique
orientation, this mapping can be used to derive the initial collision geometry.
In addition, Identikit 2 offers a robust way to measure how well a particular
model reproduces the morphology and kinematics of a pair of interacting
galaxies. A set of eight self-consistent simulations is used to demonstrate the
algorithm's ability to search a ten-dimensional parameter space and find
near-optimal matches; all eight systems are successfully reconstructed.Comment: 14 pages, 8 figures. Accepted for publication in MNRAS. To get a copy
with high-resolution figures, use the web interface, or download the
Identikit software, visit
http://www.ifa.hawaii.edu/faculty/barnes/research/identikit
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
Using tidal tails to probe dark matter halos
We use simulations of merging galaxies to explore the sensitivity of the morphology of tidal tails to variations of the halo mass distributions in the parent galaxies. Our goal is to constrain the mass of dark halos in well-known merging pairs. We concentrate on prograde encounters between equal mass galaxies which represent the best cases for creating tidal tails, but also look at systems with different relative orientations, orbital energies and mass ratios. As the mass and extent of the dark halo increase in the model galaxies, the resulting tidal tails become shorter and less massive, even under the most favorable conditions for producing these features. Our simulations imply that the observed merging galaxies with long tidal tails (\sim 50-100 kpc) such as NGC 4038/39 (the Antennae) and NGC 7252 probably have halo:disk+bulge mass ratios less than 10:1. These results conflict with the favored values of the dark halo mass of the Milky Way derived from satellite kinematics and the timing argument which give a halo:disk+bulge mass ratio of \sim 30:1. However, the lower bound of the estimated dark halo mass in the Milky Way (mass ratio \sim 10:1) is still consistent with the inferred tidal tail galaxy masses. Our results also conflict with the expectations of \Omega=1 cosmologies such as CDM which predict much more massive and extended dark halos
Figure rotation of dark halos in CDM simulations
We investigate the figure rotation of dark matter halos identified in Lambda
CDM simulations. We find that when strict criteria are used to select suitable
halos for study, 5 of the 222 halos identified in our z=0 simulation output
undergo coherent figure rotation over a 5h^{-1}Gyr period. We discuss the
effects of varying the selection criteria and find that pattern speeds for a
much larger fraction of the halos can be measured when the criteria are
relaxed. Pattern speeds measured over a 1h^{-1}Gyr period follow a log-normal
distribution, centred at Omega_p = 0.25h rad/Gyr with a maximum value of 0.94h
rad/Gyr. Over a 5h^{-1}Gyr period, the average pattern speed of a halo is about
0.1h rad/Gyr and the largest pattern speed found is 0.24h rad/Gyr. Less than
half of the selected halos showed alignment between their figure rotation axis
and minor axis, the exact fraction being somewhat dependent on how one defines
a halo. While the pattern speeds observed are lower than those generally
thought capable of causing spiral structure, we note that coherent figure
rotation is found over very long periods and argue that further simulations
would be required before strong conclusions about spiral structure in all
galaxies could be drawn. We find no correlation between halo properties such as
total mass and the pattern speed.Comment: accepted to MNRAS, 8 page
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
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 dynamics of spiral arms in pure stellar disks
It has been believed that spirals in pure stellar disks, especially the ones
spontaneously formed, decay in several galactic rotations due to the increase
of stellar velocity dispersions. Therefore, some cooling mechanism, for example
dissipational effects of the interstellar medium, was assumed to be necessary
to keep the spiral arms. Here we show that stellar disks can maintain spiral
features for several tens of rotations without the help of cooling, using a
series of high-resolution three-dimensional -body simulations of pure
stellar disks. We found that if the number of particles is sufficiently large,
e.g., , multi-arm spirals developed in an isolated disk can
survive for more than 10 Gyrs. We confirmed that there is a self-regulating
mechanism that maintains the amplitude of the spiral arms. Spiral arms increase
Toomre's of the disk, and the heating rate correlates with the squared
amplitude of the spirals. Since the amplitude itself is limited by the value of
, this makes the dynamical heating less effective in the later phase of
evolution. A simple analytical argument suggests that the heating is caused by
gravitational scattering of stars by spiral arms, and that the self-regulating
mechanism in pure-stellar disks can effectively maintain spiral arms on a
cosmological timescale. In the case of a smaller number of particles, e.g.,
, spiral arms grow faster in the beginning of the simulation
(while is small) and they cause a rapid increase of . As a result, the
spiral arms become faint in several Gyrs.Comment: 18 pages, 19 figures, accepted for Ap
The velocity peaks in the cold dark matter spectrum on Earth
The cold dark matter spectrum on earth is expected to have peaks in velocity
space. We obtain estimates for the sizes and locations of these peaks. To this
end we have generalized the secondary infall model of galactic halo formation
to include angular momentum of the dark matter particles. This new model is
still spherically symmetric and it has self-similar solutions. Our results are
relevant to direct dark matter search experiments.Comment: 12 pages including 1 table and 4 figures, LaTeX, REVTEX 3.0 versio
The effect of bars and transient spirals on the vertical heating in disk galaxies
The nature of vertical heating of disk stars in the inner as well as the
outer region of disk galaxies is studied. The galactic bar (which is the
strongest non-axisymmetric pattern in the disk) is shown to be a potential
source of vertical heating of the disk stars in the inner region. Using a
nearly self-consistent high-resolution N-body simulation of disk galaxies, the
growth rate of the bar potential is found to be positively correlated with the
vertical heating exponent in the inner region of galaxies. We also characterize
the vertical heating in the outer region where the disk dynamics is often
dominated by the presence of transient spiral waves and mild bending waves. Our
simulation results suggest that the non-axisymmetric structures are capable of
producing the anisotropic heating of the disk stars.Comment: 14 pages, 20 figures, Accepted for publication in Ap
Unfolding the Hierarchy of Voids
We present a framework for the hierarchical identification and
characterization of voids based on the Watershed Void Finder. The Hierarchical
Void Finder is based on a generalization of the scale space of a density field
invoked in order to trace the hierarchical nature and structure of cosmological
voids. At each level of the hierarchy, the watershed transform is used to
identify the voids at that particular scale. By identifying the overlapping
regions between watershed basins in adjacent levels, the hierarchical void tree
is constructed. Applications on a hierarchical Voronoi model and on a set of
cosmological simulations illustrate its potential.Comment: 5 pages, 2 figure
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