506 research outputs found
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
A multi-particle model of the 3C 48 host
The first successful multi-particle model for the host of the well-known
quasi-stellar object (QSO) 3C 48 is reported. It shows that the morphology and
the stellar velocity field of the 3C 48 host can be reproduced by the merger of
two disk galaxies. The conditions of the interaction are similar to those used
for interpreting the appearance of the ''Antennae'' (NGC 4038/39) but seen from
a different viewing angle. The model supports the controversial hypothesis that
3C 48A is the second nucleus of a merging galaxy, and it suggests a simple
solution for the problem of the missing counter tidal tail.Comment: 5 pages, 5 figures, accepted for publication 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
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
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 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
Influence of baryons on the orbital structure of dark matter haloes
We explore the dynamical signatures imprinted by baryons on dark matter
haloes during the formation process using the OverWhelmingly Large Simulations
(OWLS), a set of state-of-the-art high-resolution cosmological hydrodynamical
simulations. We present a detailed study of the effects of the implemented
feedback prescriptions on the orbits of dark matter particles, stellar
particles and subhaloes, analysing runs with no feedback, with stellar feedback
and with feedback from supermassive black holes. We focus on the central
regions (0.25 r_{200}) of haloes with virial masses ~ 6 x 10^{13} (~ 7 x
10^{11}) Msun/h at z = 0(2). We also investigate how the orbital content
(relative fractions of the different orbital types) of these haloes depends on
several key parameters such as their mass, redshift and dynamical state. The
results of spectral analyses of the orbital content of these simulations are
compared, and the change in fraction of box, tube and irregular orbits is
quantified. Box orbits are found to dominate the orbital structure of dark
matter haloes in cosmological simulations. There is a strong anticorrelation
between the fraction of box orbits and the central baryon fraction. While
radiative cooling acts to reduce the fraction of box orbits, strong feedback
implementations result in a similar orbital distribution to that of the dark
matter only case. The orbital content described by the stellar particles is
found to be remarkably similar to that drawn from the orbits of dark matter
particles, suggesting that either they have forgotten their dynamical history,
or that subhaloes bringing in stars are not biased significantly with respect
to the main distribution. The orbital content of the subhaloes is in broad
agreement with that seen in the outer regions of the particle distributions.Comment: 18 pages, 13 figures, 3 tables. Accepted for publication in MNRA
Semi-Analytical Models for the Formation of Disk Galaxies: I. Constraints from the Tully-Fisher Relation
We present new semi-analytical models for the formation of disk galaxies with
the purpose of investigating the origin of the near-infrared Tully-Fisher (TF)
relation. The models assume that disks are formed by cooling of the baryons
inside dark halos with realistic density profiles, and that the baryons
conserve their specific angular momentum. Only gas with densities above the
critical density given by Toomre's stability criterion is considered eligible
for star formation, and a simple recipe for supernovae feedback is included. We
emphasize the importance of extracting the proper luminosity and velocity
measures from the models, something that has often been ignored in the past.
The observed K-band TF relation has a slope that is steeper than simple
predictions based on dynamical arguments suggest. Taking the stability related
star formation threshold densities into account steepens the TF relation,
decreases its scatter, and yields gas mass fractions that are in excellent
agreement with observations. In order for the TF slope to be as steep as
observed, further physics are required. We argue that the characteristics of
the observed near-infrared TF relation do not reflect systematic variations in
stellar populations, or cosmological initial conditions, but are governed by
feedback. Finally we show that our models provide a natural explanation for the
small amount of scatter that makes the TF relation useful as a cosmological
distance indicator.Comment: 20 pages, 10 figures. Accepted for publication in Ap
The fate of the B ball
The gauge-mediated SUSY-breaking (GMSB) model needs entropy production at a
relatively low temperature in the thermal history of the Universe for the
unwanted relics to be diluted. This requires a mechanism for the baryogenesis
after the entropy production, and the Affleck and Dine (AD) mechanism is a
promising candidate for it. The AD baryogenesis in the GMSB model predicts the
existence of the baryonic Q ball, that is the B ball, and this may work as the
dark matter in the Universe. In this article, we discuss the stability of the B
ball in th presence of baryon-number violating interactions. We find that the
evaporation rate increases monotonically with the B-ball charge because the
large field value inside the B ball enhances the effect of the
baryon-number-violating operators. While there are some difficulties to
evaluate the evaporation rate of the B ball, we derive the evaporation time
(lifetime) of the B ball for the mass-to-charge ratio \omega_0\gsim 100 \MEV.
The lifetime of the B ball and the distortion of the cosmic ray positron flux
and the cosmic background radiation from the B ball evaporation give
constraints on the baryon number of the B ball and the interaction, if the B
ball is the dark matter. We also discuss some unresolved properties of the B
ball.Comment: 27 pages incl 8 figs, LaTe
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