279 research outputs found
Numerical calculation of linear modes in stellar disks
We present a method for solving the two-dimensional linearized collisionless
Boltzmann equation using Fourier expansion along the orbits. It resembles very
much solutions present in the literature, but it differs by the fact that
everything is performed in coordinate space instead of using action-angle
variables. We show that this approach, though less elegant, is both feasible
and straightforward. This approach is then incorporated in a matrix method in
order to calculate self-consistent modes, using a set of potential-density
pairs which is obtained numerically. We investigated the stability of some
unperturbed disks having an almost flat rotation curve, an exponential disk and
a non-zero velocity dispersion. The influence of the velocity dispersion, halo
mass and anisotropy on the stability is further discussed.Comment: 12 pages LaTeX format, uses laa.tex (enclosed), 16 PostScript
figures. tarred, gzipped, uuencoded. Postscript version available at
ftp://naos.rug.ac.be/pub/LINMOD2.ps.Z Accepted for publication in A &
On the deprojection of triaxial galaxies with St\"ackel potentials
A family of triaxial St\"ackel potential-density pairs is introduced. With
the help of a Quadratic Programming method, a linear combination of
potential-density pairs of this family which fits a given projected density
distribution can be built. This deprojection strategy can be used to model the
potentials of triaxial elliptical galaxies with or without dark halos. Besides,
we show that the expressions for the St\"ackel triaxial density and potential
are considerably simplified when expressed in terms of divided differences,
which are convenient numerically. We present an example of triaxial
deprojection for the galaxy NGC~5128 whose photometry follows the de
Vaucouleurs law.Comment: 8 pages, to appear in A&A, postscript file with figures available at
ftp://naos.rug.ac.be/pub/deprojection.ps.
A method for solving the linearized Boltzmann equation for almost uniformly rotating stellar disks.
We construct analytical phase-space solutions for perturbations of flat disks
by performing a power series expansion for the radius and the velocity
coordinates. We show that this approach translates into an elegant mathematical
formulation which is easy to use for a wide variety of distribution functions,
for as far as resonances do not play a role, such as is the case for potentials
which are close to quadratic. As a testcase, the method is applied on the
Kalnajs disks. The results obtained are in full agreement with the analytical
solutions of the mode analysis. The strongest advantages of this method are its
independence of the mathematical complexity of the unperturbed distribution,
the degree of detail with which the solutions can be calculated and its
computational straightforwardness. On the contrary, power series solutions are
not suitable for describing regions where resonant orbits occur, which we
therefore exclude in this paper. We used the technique to analyse perturbations
in the central regions of a galaxy, tracking the dynamical consequences of a
Galactic bar on the kinematics of the solar neighbourhood (Hipparcos). We
showed how the orientation and strength of the bar is related to the properties
of the velocity ellipsoid in our model.Comment: 10 pages, PostScript file including figures, to appear in Astronomy
and Astrophysic
Determination of the dynamical structure of galaxies using optical spectra
Galaxy spectra are a rich source of kinematical information since the shapes
of the absorption lines reflect the movement of stars along the line-of-sight.
We present a technique to directly build a dynamical model for a galaxy by
fitting model spectra, calculated from a dynamical model, to the observed
galaxy spectra. Using synthetic spectra from a known galaxy model we
demonstrate that this technique indeed recovers the essential dynamical
characteristics of the galaxy model. Moreover, the method allows a
statistically meaningful error analysis on the resulting dynamical quantities.Comment: 14 pages, 14 figures, Latexfile, MNRAS, in pres
Probing the halo of Centaurus A: a merger dynamical model for the PN population
Photometry and kinematics of the giant elliptical galaxy NGC~5128
(Centaurus~A) based on planetary nebulae observations (Hui~\etal 1995) are used
to build dynamical models which allow us to infer the presence of a dark matter
halo. To this end, we apply a Quadratic Programming method. Constant
mass-to-light ratio models fail to reproduce the major axis velocity dispersion
measurements at large radii: the profile of this kind of models falls off too
steeply when compared to the observations, clearly suggesting the necessity of
including a dark component in the halo. By assuming a mass-to-light ratio which
is increasing with radius, the model satisfactorily matches the observations.
The total mass for the best fit model is of which
about 50\% is dark matter. However, models with different total masses and dark
halos are also consistent with the data; we estimate that the total mass of
Cen~A within 50~kpc may vary between and
. The best fit model consists of 75\% of stars rotating
around the short axis and 25\% of stars rotating around the long axis .
Finally, the morphology of the projected velocity field is analyzed using
Statler's classification criteria (Statler 1991). We find that the appearance
of our velocity field is compatible with a type 'Nn' or 'Nd'.Comment: 13 pages, uuencoded compressed postscript, without figures. The full
postscript version, including all 14 figures, is available via anonymous ftp
at ftp://naos.rug.ac.be/pub/cena.ps.
Simulations of the formation and evolution of isolated dwarf galaxies
We present new fully self-consistent models of the formation and evolution of
isolated dwarf galaxies. We have used the publicly available N-body/SPH code
HYDRA, to which we have added a set of star formation criteria, and
prescriptions for chemical enrichment (taking into account contributions from
both SNIa and SNII), supernova feedback, and gas cooling. The models follow the
evolution of an initially homogeneous gas cloud collapsing in a pre-existing
dark-matter halo. These simplified initial conditions are supported by the
merger trees of isolated dwarf galaxies extracted from the milli-Millennium
Simulation.
The star-formation histories of the model galaxies exhibit burst-like
behaviour. These bursts are a consequence of the blow-out and subsequent
in-fall of gas. The amount of gas that leaves the galaxy for good is found to
be small, in absolute numbers, ranging between 3x10^7 Msol and 6x10^7 Msol .
For the least massive models, however, this is over 80 per cent of their
initial gas mass. The local fluctuations in gas density are strong enough to
trigger star-bursts in the massive models, or to inhibit anything more than
small residual star formation for the less massive models. Between these
star-bursts there can be time intervals of several Gyrs.
We have compared model predictions with available data for the relations
between luminosity and surface brightness profile, half-light radius, central
velocity dispersion, broad band colour (B-V) and metallicity, as well as the
location relative to the fundamental plane. The properties of the model dwarf
galaxies agree quite well with those of observed dwarf galaxies.Comment: 16 pages, 20 figures, accepted for publication in MNRA
A genetic algorithm for the non-parametric inversion of strong lensing systems
We present a non-parametric technique to infer the projected-mass
distribution of a gravitational lens system with multiple strong-lensed images.
The technique involves a dynamic grid in the lens plane on which the mass
distribution of the lens is approximated by a sum of basis functions, one per
grid cell. We used the projected mass densities of Plummer spheres as basis
functions. A genetic algorithm then determines the mass distribution of the
lens by forcing images of a single source, projected back onto the source
plane, to coincide as well as possible. Averaging several tens of solutions
removes the random fluctuations that are introduced by the reproduction process
of genomes in the genetic algorithm and highlights those features common to all
solutions. Given the positions of the images and the redshifts of the sources
and the lens, we show that the mass of a gravitational lens can be retrieved
with an accuracy of a few percent and that, if the sources sufficiently cover
the caustics, the mass distribution of the gravitational lens can also be
reliably retrieved. A major advantage of the algorithm is that it makes full
use of the information contained in the radial images, unlike methods that
minimise the residuals of the lens equation, and is thus able to accurately
reconstruct also the inner parts of the lens.Comment: 11 pages, accepted for publication by MNRA
The stability of uniformly rotating stellar disks
We explore a series expansion method to calculate the modes of oscillations for a variety of uniformly rotating finite disks, either with or without a dark halo. Since all models have the same potential, this survey focuses on the role of the distribution function in stability analyses. We show that the stability behaviour is greatly influenced by the structure of the unperturbed distribution, particularly by its energy dependence. In addition we find that uniformly rotating disks with a halo in general can feature spiral-like instabilities
Three-component Stackel potentials satisfying recent estimates of Milky Way parameters
We present a set of three-component Stackel potentials defined by five
parameters and designed to model the Milky Way. We review the fundamental
constraints that any model of the Milky Way must satisfy, including the most
recent ones derived from Hipparcos data, and we study how the parameters of the
presented potentials can vary in order to match these constraints. Five
different valid potentials are presented and analyzed in detail: they are
designed to be confronted with kinematical surveys in the future, by the
construction of three-integral analytic distribution functions.Comment: 13 pages, 5 figures, accepted for publication in MNRA
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