2,600 research outputs found
Kinematic response of the outer stellar disk to a central bar
We study, using direct orbit integrations, the kinematic response of the
outer stellar disk to the presence of a central bar, as in the Milky-Way. We
find that the bar's outer Lindblad resonance (OLR) causes significant
perturbations of the velocity moments. With increasing velocity dispersion, the
radius of these perturbations is shifted outwards, beyond the nominal position
of the OLR, but also the disk becomes less responsive. If we follow Dehnen
(2000) in assuming that the OLR occurs just inside the Solar circle and that
the Sun lags the bar major axis by ~20 degrees, we find (1) no significant
radial motion of the local standard of rest (LSR), (2) a vertex deviation of
\~10 degrees and (3) a lower ratio sigma_2/sigma_1 of the principal components
of the velocity- dispersion tensor than for an unperturbed disk. All of these
are actually consistent with the observations of the Solar-neighbourhood
kinematics. Thus it seems that at least the lowest-order deviations of the
local-disk kinematics from simple expectations based on axisymmetric
equilibrium can be attributed entirely to the influence of the Galactic bar.Comment: 10 pages, 8 figures, accepted for publication in A&
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
A Very Fast and Momentum-Conserving Tree Code
The tree code for the approximate evaluation of gravitational forces is
extended and substantially accelerated by including mutual cell-cell
interactions. These are computed by a Taylor series in Cartesian coordinates
and in a completely symmetric fashion, such that Newton's third law is
satisfied by construction and hence momentum exactly conserved. The
computational effort is further reduced by exploiting the mutual symmetry of
the interactions. For typical astrophysical problems with N=10^5 and at the
same level of accuracy, the new code is about four times faster than the tree
code. For large N, the computational costs are found to scale almost linearly
with N, which can also be supported by a theoretical argument, and the
advantage over the tree code increases with ever larger N.Comment: revised version (accepted by ApJ Letters), 5 pages LaTeX, 3 figure
On the coupling of massless particles to scalar fields
It is investigated if massless particles can couple to scalar fields in a
special relativistic theory with classical particles. The only possible obvious
theory which is invariant under Lorentz transformations and reparametrization
of the affine parameter leads to trivial trajectories (straight lines) for the
massless case, and also the investigation of the massless limit of the massive
theory shows that there is no influence of the scalar field on the limiting
trajectories.
On the other hand, in contrast to this result, it is shown that massive
particles are influenced by the scalar field in this theory even in the
ultra-relativistic limit.Comment: 9 pages, no figures, uses titlepage.sty, LaTeX 2.09 file, submitted
to International Journal of Theoretical Physic
A fast multipole method for stellar dynamics
The approximate computation of all gravitational forces between
interacting particles via the fast multipole method (FMM) can be made as
accurate as direct summation, but requires less than
operations. FMM groups particles into spatially bounded cells and uses
cell-cell interactions to approximate the force at any position within the sink
cell by a Taylor expansion obtained from the multipole expansion of the source
cell. By employing a novel estimate for the errors incurred in this process, I
minimise the computational effort required for a given accuracy and obtain a
well-behaved distribution of force errors. For relative force errors of
, the computational costs exhibit an empirical scaling of . My implementation (running on a 16 core node) out-performs a
GPU-based direct summation with comparable force errors for .Comment: 21 pages, 15 figures, accepted for publication in Journal for
Computational Astrophysics and Cosmolog
Tracing the Hercules stream around the Galaxy
It has been proposed that the Hercules stream, a group of co-moving stars in
the Solar neighborhood offset from the bulk of the velocity distribution, is
the result of resonant interactions between stars in the outer disk and the
Galactic bar. So far it has only been seen in the immediate Solar neighborhood,
but the resonance model makes a prediction over a large fraction of the
Galactic disk. I predict the distribution of stellar velocities and the
changing Hercules feature in this distribution as a function of location in the
Galactic disk in a simple model for the Galaxy and the bar that produces the
observed Hercules stream. The Hercules feature is expected to be strong enough
to be unambiguously detected in the distribution of line-of-sight velocities in
selected directions. I identify quantitatively the most promising lines of
sight for detection in line-of-sight velocities using the Kullback-Leibler
divergence between the predictions of the resonance model and an axisymmetric
model; these directions are at 250 deg <~ l <~ 290 deg. The predictions
presented here are only weakly affected by distance uncertainties, assumptions
about the distribution function in the stellar disk, and the details of the
Galactic potential including the effect of spiral structure. Gaia and future
spectroscopic surveys of the Galactic disk such as APOGEE and HERMES will be
able to robustly test the origin of the Hercules stream and constrain the
properties of the Galactic bar
Black hole foraging: feedback drives feeding
We suggest a new picture of supermassive black hole (SMBH) growth in galaxy
centers. Momentum-driven feedback from an accreting hole gives significant
orbital energy but little angular momentum to the surrounding gas. Once central
accretion drops, the feedback weakens and swept-up gas falls back towards the
SMBH on near-parabolic orbits. These intersect near the black hole with
partially opposed specific angular momenta, causing further infall and
ultimately the formation of a small-scale accretion disk. The feeding rates
into the disk typically exceed Eddington by factors of a few, growing the hole
on the Salpeter timescale and stimulating further feedback. Natural
consequences of this picture include (i) the formation and maintenance of a
roughly toroidal distribution of obscuring matter near the hole; (ii) random
orientations of successive accretion disk episodes; (iii) the possibility of
rapid SMBH growth; (iv) tidal disruption of stars and close binaries formed
from infalling gas, resulting in visible flares and ejection of hypervelocity
stars; (v) super-solar abundances of the matter accreting on to the SMBH; and
(vi) a lower central dark-matter density, and hence annihilation signal, than
adiabatic SMBH growth implies. We also suggest a simple sub-grid recipe for
implementing this process in numerical simulations.Comment: accepted for publication in ApJ Letters, 5 pages, 1 figur
Dynamical Models for the Milky Way
The only way to map the Galaxy's gravitational potential and
the distribution of matter that produces it is by modelling the dynamics of
stars and gas. Observations of the kinematics of gas provide key information
about gradients of within the plane, but little information about the
structure of out of the plane. Traditional Galaxy models {\em assume},
for each of the Galaxy's components, arbitrary flattenings, which together with
the components' relative masses yield the model's equipotentials. However, the
Galaxy's isopotential surfaces should be {\em determined\/} directly from the
motions of stars that move far from the plane. Moreover, from the kinematics of
samples of such stars that have well defined selection criteria, one should be
able not only to map at all positions, but to determine the distribution
function of each stellar population studied. These
distribution functions will contain a wealth of information relevant to the
formation and evolution of the Galaxy. An approach to fitting a wide class of
dynamical models to the very heterogeneous body of available data is described
and illustrated.Comment: 10 pages, LaTeX, style file and 4 figures included. Invited talk
presented at the meeting ``Formation of the Galactic Halo ... Inside and
Out'', Tucson, October 9-11. Full .ps file available at
ftp://ftp.physics.ox.ac.uk/pub/local/users/dehnen/MilkyWayModels.ps.g
- …