66 research outputs found
Actions for axisymmetric potentials
We give an algorithm for the economical calculation of angles and actions for
stars in axisymmetric potentials. We test the algorithm by integrating orbits
in a realistic model of the Galactic potential, and find that, even for orbits
characteristic of thick-disc stars, the errors in the actions are typically
smaller than 2 percent. We describe a scheme for obtaining actions by
interpolation on tabulated values that significantly accelerates the process of
calculating observables quantities, such as density and velocity moments, from
a distribution function.Comment: 5 pages accepted for publication in MNRA
Probing a regular orbit with spectral dynamics
We have extended the spectral dynamics formalism introduced by Binney &
Spergel, and have implemented a semi-analytic method to represent regular
orbits in any potential, making full use of their regularity. We use the
spectral analysis code of Carpintero & Aguilar to determine the nature of an
orbit (irregular, regular, resonant, periodic) from a short-time numerical
integration. If the orbit is regular, we approximate it by a truncated Fourier
time series of a few tens of terms per coordinate. Switching to a description
in action-angle variables, this corresponds to a reconstruction of the
underlying invariant torus. We then relate the uniform distribution of a
regular orbit on its torus to the non-uniform distribution in the space of
observables by a simple Jacobian transformation between the two sets of
coordinates. This allows us to compute, in a cell-independent way, all the
physical quantities needed in the study of the orbit, including the density and
in the line-of-sight velocity distribution, with much increased accuracy. The
resulting flexibility in the determination of the orbital properties, and the
drastic reduction of storage space for the orbit library, provide a significant
improvement in the practical application of Schwarzschild's orbit superposition
method for constructing galaxy models. We test and apply our method to
two-dimensional orbits in elongated discs, and to the meridional motion in
axisymmetric potentials, and show that for a given accuracy, the spectral
dynamics formalism requires an order of magnitude fewer computations than the
more traditional approaches.Comment: 13 pages, 18 eps figures, submitted to MNRA
Origin and possible birthplace of the extreme runaway star HIP 60350
Using the recently determined spatial velocity components of the extreme
runaway star HIP 60350 and a gravitation potential model of the Galaxy, we
integrate the orbit of HIP 60350 back to the plane of the Galaxy. In this way,
a possible location of the formation of the star is determined. We estimate the
uncertainty of the result due to the uncertainties of the gravitational
potential model and the errors in the spatial velocity components. The place of
birth lies (within the errors) near the position of the open cluster NGC 3603.
However, the ejection event which occured about 20 Myr ago is in contradiction
with the cluster mean age of 3-4 Myr. We suggest that it occured at an earlier
phase in sequential star formation in that region. We discuss also ejection
mechanisms. Due to the rather high mass of the star (about 5 \Msol), the most
probable model is that of dynamical ejection.Comment: Astron. Astrophys. submitted, 7 pages, 5 eps figure
Difficulties with Recovering The Masses of Supermassive Black Holes from Stellar Kinematical Data
We investigate the ability of three-integral, axisymmetric, orbit-based
modeling algorithms to recover the parameters defining the gravitational
potential (M/L ratio and black hole mass Mh) in spheroidal stellar systems
using stellar kinematical data. We show that the potential estimation problem
is generically under-determined when applied to long-slit kinematical data of
the kind used in most black hole mass determinations to date. A range of
parameters (M/L, Mh) can provide equally good fits to the data, making it
impossible to assign best-fit values. We illustrate the indeterminacy using a
variety of data sets derived from realistic models as well as published
observations of the galaxy M32. In the case of M32, our reanalysis demonstrates
that data published prior to 2000 are equally consistent with Mh in the range
1.5x10^6-5x10^6 solar masses, with no preferred value in that range. While the
HST/STIS data for this galaxy may overcome the degeneracy in Mh, HST data for
most galaxies do not resolve the black hole's sphere of influence and in these
galaxies the degree of degeneracy allowed by the data may be substantial. We
investigate the effect on the degeneracy of enforcing smoothness
(regularization) constraints. However we find no indication that the true
potential can be recovered simply by enforcing smoothness. For a given
smoothing level, all solutions in the minimum-chisquare valley exhibit similar
levels of noise. These experiments affirm that the indeterminacy is real and
not an artifact associated with non-smooth solutions. (Abridged)Comment: Accepted for publication in The Astrophysical Journal. Changes
include discussion of regularizatio
ON the CONSERVATION of the VERTICAL ACTION in GALACTIC DISKS
We employ high-resolution N-body simulations of isolated spiral galaxy models, from low-amplitude, multi-armed galaxies to Milky Way-like disks, to estimate the vertical action of ensembles of stars in an axisymmetrical potential. In the multi-armed galaxy the low-amplitude arms represent tiny perturbations of the potential, hence the vertical action for a set of stars is conserved, although after several orbital periods of revolution the conservation degrades significantly. For a Milky Way-like galaxy with vigorous spiral activity and the formation of a bar, our results show that the potential is far from steady, implying that the action is not a constant of motion. Furthermore, because of the presence of high-amplitude arms and the bar, considerable in-plane and vertical heating occurs that forces stars to deviate from near-circular orbits, reducing the degree at which the actions are conserved for individual stars, in agreement with previous results, but also for ensembles of stars. If confirmed, this result has several implications, including the assertion that the thick disk of our Galaxy forms by radial migration of stars, under the assumption of the conservation of the action describing the vertical motion of stars. © 2016. The American Astronomical Society. All rights reserved
Relativistic and Newtonian core-shell models: analytical and numerical results
We make a detailed analysis of Newtonian as well as relativistic core-shell
models recently proposed to describe a black hole or neutron star surrounded by
shells of matter, and in a seminal sense also galaxies, supernovae and star
remnants since there are massive shell-like structures surrounding many of them
and also evidences for many galactic nuclei hiding black holes. We discuss the
unicity of the models in relation to their analyticity at the black hole
horizon and also to the full elimination of conical singularities. Secondly, we
study the role played by the presence/lack of discrete reflection symmetries
about equatorial planes in the chaotic behavior of the orbits, which is to be
contrasted with the almost universal acceptance of reflection symmetries as
default assumptions in galactic modeling. We also compare the related effects
if we change a true central black hole by a Newtonian central mass. The
numerical findings are: 1- The breakdown of the reflection symmetry about the
equatorial plane in both Newtonian and relativistic core-shell models does i)
enhance in a significant way the chaoticity of orbits in reflection symmetric
oblate shell models and ii) inhibit significantly also the occurrence of chaos
in reflection symmetric prolate shell models. In particular, in the prolate
case the lack of the reflection symmetry provides the phase space with a robust
family of regular orbits that is otherwise not found at higher energies. 2- The
relative extents of the chaotic regions in the relativistic cases (i. e. with a
true central black hole) are significantly larger than in the corresponding
Newtonian ones (which have just a central potential).Comment: AASTEX, 22 pages plus 28 postscript figures, to appear in Ap.
Galactic kinematics with modified Newtonian dynamics
We look for observational signatures that could discriminate between
Newtonian and modified Newtonian (MOND) dynamics in the Milky Way, in view of
the advent of large astrometric and spectroscopic surveys. Indeed, a typical
signature of MOND is an apparent disk of "phantom" dark matter, which is
uniquely correlated with the visible disk-density distribution. Due to this
phantom dark disk, Newtonian models with a spherical halo have different
signatures from MOND models close to the Galactic plane. The models can thus be
differentiated by measuring dynamically (within Newtonian dynamics) the disk
surface density at the solar radius, the radial mass gradient within the disk,
or the velocity ellipsoid tilt angle above the Galactic plane. Using the most
realistic possible baryonic mass model for the Milky Way, we predict that, if
MOND applies, the local surface density measured by a Newtonist will be
approximately 78 Msun/pc2 within 1.1 kpc of the Galactic plane, the dynamically
measured disk scale-length will be enhanced by a factor of 1.25 with respect to
the visible disk scale-length, and the local vertical tilt of the velocity
ellipsoid at 1 kpc above the plane will be approximately 6 degrees. None of
these tests can be conclusive for the present-day accuracy of Milky Way data,
but they will be of prime interest with the advent of large surveys such as
GAIA.Comment: 5 page
Kinematical and chemical vertical structure of the Galactic thick disk I. Thick disk kinematics
The variation of the kinematical properties of the Galactic thick disk with
Galactic height Z are studied by means of 412 red giants observed in the
direction of the south Galactic pole up to 4.5 kpc from the plane. We confirm
the non-null mean radial motion toward the Galactic anticenter found by other
authors, but we find that it changes sign at |Z|=3 kpc, and the proposed inward
motion of the LSR alone cannot explain these observations. The rotational
velocity decreases with |Z| by -30 km/s/kpc, but the data are better
represented by a power-law with index 1.25, similar to that proposed from the
analysis of SDSS data. All the velocity dispersions increase with |Z|, but the
vertical gradients are small. The dispersions grow proportionally, with no
significant variation of the anisotropy. The ratio sigma_U/sigma_W=2 suggests
that the thick disk could have formed from a low-latitude merging event. The
vertex deviation increases with Galactic height, reaching ~20 degrees at
|Z|=3.5 kpc. The tilt angle also increases, and the orientation of the
ellipsoid in the radial-vertical plane is constantly intermediate between the
alignment with the cylindrical and the spherical coordinate systems. The tilt
angle at |Z|=2 kpc coincides with the expectations of MOND, but an extension of
the calculations to higher |Z| is required to perform a conclusive test.
Finally, between 2.5 and 3.5 kpc we detect deviations from the linear trend of
many kinematical quantities, suggesting that some kinematical substructure
could be present.Comment: Accepted for publication in Ap
Axisymmetric Three-Integral Models for Galaxies
We describe an improved, practical method for constructing galaxy models that
match an arbitrary set of observational constraints, without prior assumptions
about the phase-space distribution function (DF). Our method is an extension of
Schwarzschild's orbit superposition technique. As in Schwarzschild's original
implementation, we compute a representative library of orbits in a given
potential. We then project each orbit onto the space of observables, consisting
of position on the sky and line-of-sight velocity, while properly taking into
account seeing convolution and pixel binning. We find the combination of orbits
that produces a dynamical model that best fits the observed photometry and
kinematics of the galaxy. A key new element of this work is the ability to
predict and match to the data the full line-of-sight velocity profile shapes. A
dark component (such as a black hole and/or a dark halo) can easily be included
in the models.
We have tested our method, by using it to reconstruct the properties of a
two-integral model built with independent software. The test model is
reproduced satisfactorily, either with the regular orbits, or with the
two-integral components. This paper mainly deals with the technical aspects of
the method, while applications to the galaxies M32 and NGC 4342 are described
elsewhere (van der Marel et al., Cretton & van den Bosch). (abridged)Comment: minor changes, accepted for publication in the Astrophysical Journal
Supplement
Triaxial orbit based galaxy models with an application to the (apparent) decoupled core galaxy NGC 4365
We present a flexible and efficient method to construct triaxial dynamical
models of galaxies with a central black hole, using Schwarzschild's orbital
superposition approach. Our method is general and can deal with realistic
luminosity distributions, which project to surface brightness distributions
that may show position angle twists and ellipticity variations. The models are
fit to measurements of the full line-of-sight velocity distribution (wherever
available). We verify that our method is able to reproduce theoretical
predictions of a three-integral triaxial Abel model. In a companion paper (van
de Ven, de Zeeuw & van den Bosch), we demonstrate that the method recovers the
phase-space distribution function. We apply our method to two-dimensional
observations of the E3 galaxy NGC 4365, obtained with the integral-field
spectrograph SAURON, and study its internal structure, showing that the
observed kinematically decoupled core is not physically distinct from the main
body and the inner region is close to oblate axisymmetric.Comment: 21 Pages, 14 (Colour) Figures, Companion paper is arXiv:0712.0309
Accepted to MNRAS. Full resolution version at
http://www.strw.leidenuniv.nl/~bosch/papers/RvdBosch_triaxmethod.pd
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