5,654 research outputs found
Applying Schwarzschild's orbit superposition method to barred or non-barred disc galaxies
We present an implementation of the Schwarzschild orbit superposition method
which can be used for constructing self-consistent equilibrium models of barred
or non-barred disc galaxies, or of elliptical galaxies with figure rotation.
This is a further development of the publicly available code SMILE; its main
improvements include a new efficient representation of an arbitrary
gravitational potential using two-dimensional spline interpolation of Fourier
coefficients in the meridional plane, as well as the ability to deal with
rotation of the density profile and with multicomponent mass models. We compare
several published methods for constructing composite axisymmetric
disc--bulge--halo models and demonstrate that our code produces the models that
are closest to equilibrium. We also apply it to create models of triaxial
elliptical galaxies with cuspy density profiles and figure rotation, and find
that such models can be found and are stable over many dynamical times in a
wide range of pattern speeds and angular momenta, covering both slow- and
fast-rotator classes. We then attempt to create models of strongly barred disc
galaxies, using an analytic three-component potential, and find that it is not
possible to make a stable dynamically self-consistent model for this density
profile. Finally, we take snapshots of two N-body simulations of barred disc
galaxies embedded in nearly-spherical haloes, and construct equilibrium models
using only information on the density profile of the snapshots. We demonstrate
that such reconstructed models are in near-stationary state, in contrast with
the original N-body simulations, one of which displayed significant secular
evolution.Comment: 15 pages, 9 figures; MNRAS, 450, 2842. The software is available at
http://td.lpi.ru/~eugvas/smile
Modelling the Galaxy for GAIA
Techniques for the construction of dynamical Galaxy models should be
considered essential infrastructure that should be put in place before GAIA
flies. Three possible modelling techniques are discussed. Although one of these
seems to have significantly more potential than the other two, at this stage
work should be done on all three.
A major effort is needed to decide how to make a model consistent with a
catalogue such as that which GAIA will produce. Given the complexity of the
problem, it is argued that a hierarchy of models should be constructed, of ever
increasing complexity and quality of fit to the data. The potential that
resonances and tidal streams have to indicate how a model should be refined is
briefly discussed.Comment: 7 pages to appear in The Three Dimensional Universe with GAIA, eds M.
Perryman & C. Turo
SKIRT: the design of a suite of input models for Monte Carlo radiative transfer simulations
The Monte Carlo method is the most popular technique to perform radiative
transfer simulations in a general 3D geometry. The algorithms behind and
acceleration techniques for Monte Carlo radiative transfer are discussed
extensively in the literature, and many different Monte Carlo codes are
publicly available. On the contrary, the design of a suite of components that
can be used for the distribution of sources and sinks in radiative transfer
codes has received very little attention. The availability of such models, with
different degrees of complexity, has many benefits. For example, they can serve
as toy models to test new physical ingredients, or as parameterised models for
inverse radiative transfer fitting. For 3D Monte Carlo codes, this requires
algorithms to efficiently generate random positions from 3D density
distributions. We describe the design of a flexible suite of components for the
Monte Carlo radiative transfer code SKIRT. The design is based on a combination
of basic building blocks (which can be either analytical toy models or
numerical models defined on grids or a set of particles) and the extensive use
of decorators that combine and alter these building blocks to more complex
structures. For a number of decorators, e.g. those that add spiral structure or
clumpiness, we provide a detailed description of the algorithms that can be
used to generate random positions. Advantages of this decorator-based design
include code transparency, the avoidance of code duplication, and an increase
in code maintainability. Moreover, since decorators can be chained without
problems, very complex models can easily be constructed out of simple building
blocks. Finally, based on a number of test simulations, we demonstrate that our
design using customised random position generators is superior to a simpler
design based on a generic black-box random position generator.Comment: 15 pages, 4 figures, accepted for publication in Astronomy and
Computin
Magnetic fields in ring galaxies
Many galaxies contain magnetic fields supported by galactic dynamo action.
However, nothing definitive is known about magnetic fields in ring galaxies.
Here we investigate large-scale magnetic fields in a previously unexplored
context, namely ring galaxies, and concentrate our efforts on the structures
that appear most promising for galactic dynamo action, i.e. outer star-forming
rings in visually unbarred galaxies. We use tested methods for modelling
galactic dynamos, taking into account the available
observational information concerning ionized interstellar matter in ring
galaxies. Our main result is that dynamo drivers in ring galaxies are strong
enough to excite large-scale magnetic fields in the ring galaxies studied. The
variety of dynamo driven magnetic configurations in ring galaxies obtained in
our modelling is much richer than that found in classical spiral galaxies. In
particular, various long-lived transients are possible. An especially
interesting case is that of NGC 4513 where the ring counter-rotates with
respect to the disc. Strong shear in the region between the disc and the ring
is associated with unusually strong dynamo drivers for the counter-rotators.
The effect of the strong drivers is found to be unexpectedly moderate. With
counter-rotation in the disc, a generic model shows that a steady mixed parity
magnetic configuration, unknown for classical spiral galaxies, may be excited,
although we do not specifically model NGC 4513. We deduce that ring galaxies
constitute a morphological class of galaxies in which identification of
large-scale magnetic fields from observations of polarized radio emission, as
well as dynamo modelling, may be possible. Such studies have the potential to
throw additional light on the physical nature of rings, their lifetimes and
evolution.Comment: 25 pages, 12 figure
Modelling Galaxies with f(E,Lz); a Black Hole in M32
A technique for the construction of axisymmetric distribution functions for
individual galaxies is presented. It starts from the observed surface bright-
ness distribution, which is deprojected to gain the axisymmetric luminosity
density, from which follows the stars' gravitational potential. After adding
dark mass components, such as a central black hole, the two-integral distribu-
tion function (2I-DF) f(E,Lz), which depends only on the classical integrals of
motion in an axisymmetric potential, is constructed using the Richardson- Lucy
algorithm. This algorithm proved to be very efficient in finding f(E,Lz)
provided the integral equation to be solved has been properly modified. Once
the 2I-\df\ is constructed, its kinematics can be computed and compared with
those observed. Many discrepancies may be remedied by altering the assumed
inclination angle, mass-to-light ratio, dark components, and odd part of the
2I-DF. Remaining discrepancies may indicate, that the distribution function
depends on the non-classical third integral, or is non-axisymmetric.
The method has been applied to the nearby elliptical galaxy M32. A 2I-DF with
~55 degrees inclination and a central black hole (or other compact dark mass
inside ~1pc) of 1.6-2*10^6 Msun fits the high-spatial-resolution kinema- tic
data of van der Marel et al. remarkably well. 2I-DFs with a significantly less
or more massive central dark mass or with edge-on inclination can be ruled out
for M32. Predictions are made for HST-observations: spectroscopy using its
smallest square aperture of 0.09"*0.09" should yield a non-gaussian central
velocity profile with broad wings, true and gaussian-fit velocity dispersion of
150-170km/s and 120-130km/s, respectively.Comment: 14 pages, 9 figures, uuencoded compressed ps file (468k), Ref:
OUTP-94-04
Magnetic topologies of cool stars
Stellar magnetic fields can be investigated using several, very complementary
approaches. While conventional spectroscopy is capable of estimating the
average magnetic strength of potentially complex field configurations thanks to
its low sensitivity to the vector properties of the field, spectropolarimetry
can be used to map the medium- and large-scale structure of magnetic
topologies. In particular, the latter approach allows one to retrieve
information about the poloidal and toroidal components of the large-scale
dynamo fields in low-mass stars, and thus to investigate the physical processes
that produce them. Similarly, this technique can be used to explore how
magnetic fields couple young stars to their massive accretion disc and thus to
estimate how much mass and angular momentum are transfered to the newly-born
low-mass star. We present here the latest results in this field obtained with
spectropolarimetry, with special emphasis on the surprising discoveries
obtained on very-low mass fully-convective stars and classical T Tauri stars
thanks to the ESPaDOnS spectropolarimeter recently installed on the 3.6m
Canada-France-Hawaii Telescope.Comment: 10p invited review paper, 3 figures, to be published in the
proceedings of the 14th Cambridge Workshop on Cool Stars, Stellar Systems,
and the Sun, November 6-10, 2006, ed. G. van Belle (ASP Conf Ser
Dynamics of Barred Galaxies
Some 30% of disc galaxies have a pronounced central bar feature in the disc
plane and many more have weaker features of a similar kind. Kinematic data
indicate that the bar constitutes a major non-axisymmetric component of the
mass distribution and that the bar pattern tumbles rapidly about the axis
normal to the disc plane. The observed motions are consistent with material
within the bar streaming along highly elongated orbits aligned with the
rotating major axis. A barred galaxy may also contain a spheroidal bulge at its
centre, spirals in the outer disc and, less commonly, other features such as a
ring or lens. Mild asymmetries in both the light and kinematics are quite
common. We review the main problems presented by these complicated dynamical
systems and summarize the effort so far made towards their solution,
emphasizing results which appear secure. (Truncated)Comment: This old review appeared in 1993. Plain tex with macro file. 82 pages
18 figures. A pdf version with figures at full resolution (3.24MB) is
available at http://www.physics.rutgers.edu/~sellwood/bar_review.pd
Magnetic fields from low mass stars to brown dwarfs
Magnetic fields have been detected on stars across the H-R diagram and
substellar objects either directly by their effect on the formation of spectral
lines, or through the activity phenomena they power which can be observed
across a large part of the electromagnetic spectrum. Stars show a very wide
variety of magnetic properties in terms of strength, geometry or variability.
Cool stars generate their magnetic fields by dynamo effect, and their
properties appear to correlate - to some extent - with stellar parameters such
as mass, rotation and age. With the improvements of instrumentation and data
analysis techniques, magnetic fields can now be detected and studied down to
the domain of very-low-mass stars and brown dwarfs, triggering new theoretical
works aimed, in particular, at modelling dynamo action in these objects. After
a brief discussion on the importance of magnetic field in stellar physics, the
basics of dynamo theory and magnetic field measurements are presented. The main
results stemming from observational and theoretical studies of magnetism are
then detailed in two parts: the fully-convective transition, and the very-low
mass stars and brown dwarfs domain.Comment: 30 pages, 9 figures. Notes for lectures presented at the Evry
Schatzman school on "Low-mass stars and the transition from stars to brown
dwarfs", September 2011, Roscoff, France. To appear in the EAS Conference
Series, edited by C. Charbonnel, C. Reyle, M. Schulthei
- …