500 research outputs found
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&
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
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
Equilibrium Disk-Bulge-Halo Models for the Milky Way and Andromeda Galaxies
We describe a new set of self-consistent, equilibrium disk galaxy models that
incorporate an exponential disk, a Hernquist model bulge, an NFW halo and a
central supermassive black hole. The models are derived from explicit
distribution functions for each component and the large number of parameters
permit detailed modeling of actual galaxies. We present techniques that use
structural and kinematic data such as radial surface brightness profiles,
rotation curves and bulge velocity dispersion profiles to find the best-fit
models for the Milky Way and M31. Through N-body realizations of these models
we explore their stability against the formation of bars. The models permit the
study of a wide range of dynamical phenomenon with a high degree of realism.Comment: 58 pages, 20 figures, submitted to the Astrophysical Journa
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
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
Tidal tails in CDM cosmologies
We study the formation of tidal tails in pairs of merging disk galaxies with
structural properties motivated by current theories of cold dark matter (CDM)
cosmologies. In a recent study, Dubinski, Mihos & Hernquist (1996) showed that
the formation of prominent tidal tails can be strongly suppressed by massive
and extended dark haloes. For the large halo-to-disk mass ratio expected in CDM
cosmologies their sequence of models failed to produce strong tails like those
observed in many well-known pairs of interacting galaxies. In order to test
whether this effect can constrain the viability of CDM cosmologies, we
construct N-body models of disk galaxies with structural properties derived in
analogy to the analytical work of Mo, Mao & White (1998). With a series of
self-consistent collisionless simulations of galaxy-galaxy mergers we
demonstrate that even the disks of very massive dark haloes have no problems
developing long tidal tails, provided the halo spin parameter is large enough.
We show that the halo-to-disk mass ratio is a poor indicator for the ability to
produce tails. Instead, the relative size of disk and halo, or alternatively,
the ratio of circular velocity to local escape speed at the half mass radius of
the disk are more useful criteria. This result holds in all CDM cosmologies.
The length of tidal tails is thus unlikely to provide useful constraints on
such models.Comment: 17 pages, mn.sty, 13 included eps-figures, submitted to MNRA
PAndAS in the mist: The stellar and gaseous mass within the halos of M31 and M33
Large scale surveys of the prominent members of the Local Group have provided
compelling evidence for the hierarchical formation of massive galaxies,
revealing a wealth of substructure that is thought to be the debris from
ancient and on-going accretion events. In this paper, we compare two extant
surveys of the M31-M33 subgroup of galaxies; the Pan-Andromeda Archaeological
Survey (PAndAS) of the stellar structure, and a combination of observations of
the HI gaseous content, detected at 21cm. Our key finding is a marked lack of
spatial correlation between these two components on all scales, with only a few
potential overlaps between stars and gas.The paucity of spatial correlation
significantly restricts the analysis of kinematic correlations, although there
does appear to the HI kinematically associated with the Giant Stellar Stream
where it passes the disk of M31. These results demonstrate that that different
processes must significantly influence the dynamical evolution of the stellar
and HI components of substructures, such as ram pressure driving gas away from
a purely gravitational path. Detailed modelling of the offset between the
stellar and gaseous substructure will provide a determination of the properties
of the gaseous halo of M31 and M33.Comment: 11 pages, 6 figures. Accepted for publication in the Astrophysical
Journal. Figure quality reduced. High quality version available at
http://www.physics.usyd.edu.au/~gfl/Arxiv_Papers/PAndAS_Mist
UGC 7388: a galaxy with two tidal loops
We present the results of spectroscopic and morphological studies of the
galaxy UGC7388 with the 8.1-m Gemini North telescope. Judging by its observed
characteristics, UGC7388 is a giant late-type spiral galaxy seen almost
edge-on. The main body of the galaxy is surrounded by two faint (\mu(B) ~ 24
and \mu(B) ~ 25.5) extended (~20-30 kpc) loop-like structures. A large-scale
rotation of the brighter loop about the main galaxy has been detected. We
discuss the assumption that the tidal disruption of a relatively massive
companion is observed in the case of UGC7388. A detailed study and modeling of
the observed structure of this unique galaxy can give important information
about the influence of the absorption of massive companions on the galactic
disks and about the structure of the dark halo around UGC7388.Comment: 8 pages, 5 figure
Parallelization, Special Hardware and Post-Newtonian Dynamics in Direct N - Body Simulations
The formation and evolution of supermassive black hole (SMBH) binaries during and after galaxy mergers is an important ingredient for our understanding of galaxy formation and evolution in a cosmological context, e.g. for predictions of cosmic star formation histories or of SMBH demographics (to predict events that emit gravitational waves). If galaxies merge in the course of their evolution, there should be either many binary or even multiple black holes, or we have to find out what happens to black hole multiples in galactic nuclei, e.g. whether they come sufficiently close to merge resulting from emission of gravitational waves, or whether they eject each other in gravitational slingshot interactions
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