723 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&

### 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

### The Galactic Kinematics of Mira Variables

The galactic kinematics of Mira variables derived from radial velocities,
Hipparcos proper motions and an infrared period-luminosity relation are
reviewed. Local Miras in the 145-200day period range show a large asymmetric
drift and a high net outward motion in the Galaxy. Interpretations of this
phenomenon are considered and (following Feast and Whitelock 2000) it is
suggested that they are outlying members of the bulge-bar population and
indicate that this bar extends beyond the solar circle.Comment: 7 pages, 2 figure, to be published in Mass-Losing Pulsating Stars and
their Circumstellar Matter, Y. Nakada & M. Honma (eds) Kluwer ASSL serie

### Understanding the assembly of Kepler's compact planetary systems

The Kepler mission has recently discovered a number of exoplanetary systems,
such as Kepler-11 and Kepler-32, in which ensembles of several planets are
found in very closely packed orbits (often within a few percent of an AU of one
another). These compact configurations present a challenge for traditional
planet formation and migration scenarios. We present a dynamical study of the
assembly of these systems, using an N-body method which incorporates a
parametrized model of planet migration in a turbulent protoplanetary disc. We
explore a wide parameter space, and find that under suitable conditions it is
possible to form compact, close-packed planetary systems via traditional
disc-driven migration. We find that simultaneous migration of multiple planets
is a viable mechanism for the assembly of tightly-packed planetary systems, as
long as the disc provides significant eccentricity damping and the level of
turbulence in the disc is modest. We discuss the implications of our preferred
parameters for the protoplanetary discs in which these systems formed, and
comment on the occurrence and significance of mean-motion resonances in our
simulations.Comment: 12 pages, 4 figures, 2 tables. Accepted for publication in Monthly
Notices of the Royal Astronomical Societ

### Dynamics of the Narrow-Line Region in the Seyfert 2 Galaxy NGC 1068

We present dynamical models based on a study of high-resolution long-slit
spectra of the narrow-line region (NLR) in NGC 1068 obtained with the Space
Telescope Imaging Spectrograph (STIS) aboard The Hubble Space Telescope (HST).
The dynamical models consider the radiative force due to the active galactic
nucleus (AGN), gravitational forces from the supermassive black hole (SMBH),
nuclear stellar cluster, and galactic bulge, and a drag force due to the NLR
clouds interacting with a hot ambient medium. The derived velocity profile of
the NLR gas is compared to that obtained from our previous kinematic models of
the NLR using a simple biconical geometry for the outflowing NLR clouds. The
results show that the acceleration profile due to radiative line driving is too
steep to fit the data and that gravitational forces along cannot slow the
clouds down, but with drag forces included, the clouds can slow down to the
systemic velocity over the range 100--400 pc, as observed. However, we are not
able to match the gradual acceleration of the NLR clouds from ~0 to ~100 pc,
indicating the need for additional dynamical studies.Comment: Paper prepared by emulateapj version 10/09/06 and accepted for print
in Ap

### Deprojection of Rich Cluster Images

We consider a general method of deprojecting 2D images to reconstruct the 3D
structure of the projected object, assuming axial symmetry. The method consists
of the application of the Fourier Slice Theorem to the general case where the
axis of symmetry is not necessarily perpendicular to the line of sight, and is
based on an extrapolation of the image Fourier transform into the so-called
cone of ignorance. The method is specifically designed for the deprojection of
X-ray, Sunyaev-Zeldovich (SZ) and gravitational lensing maps of rich clusters
of galaxies. For known values of the Hubble constant, H0, and inclination
angle, the quality of the projection depends on how exact is the extrapolation
in the cone of ignorance. In the case where the axis of symmetry is
perpendicular to the line of sight and the image is noise-free, the
deprojection is exact. Given an assumed value of H0, the inclination angle can
be found by matching the deprojected structure out of two different images of a
given cluster, e.g., SZ and X-ray maps. However, this solution is degenerate
with respect to its dependence on the assumed H0, and a third independent image
of the given cluster is needed to determine H0 as well. The application of the
deprojection algorithm to upcoming SZ, X-ray and weak lensing projected mass
images of clusters will serve to determine the structure of rich clusters, the
value of H0, and place constraints on the physics of the intra-cluster gas and
its relation to the total mass distribution.Comment: 7 pages, LaTeX, 2 Postscript figures, uses as2pp4.sty. Accepted for
publication in ApJ Letters. Also available at:
http://astro.berkeley.edu:80/~squires/papers/deproj.ps.g

### Cusp Disruption in Minor Mergers

We present 0.55 x 10^6 particle simulations of the accretion of high-density
dwarf galaxies by low-density giant galaxies, using models that contain both
power-law central density cusps and point masses representing supermassive
black holes. The cusp of the dwarf galaxy is disrupted during the merger,
producing a remnant with a central density that is only slightly higher than
that of the giant galaxy initially. Removing the black hole from the giant
galaxy allows the dwarf galaxy to remain intact and leads to a remnant with a
high central density, contrary to what is observed. Our results support the
hypothesis that the persistence of low-density cores in giant galaxies is a
consequence of supermassive black holes.Comment: 5 pages, 2 postscript figures, uses emulateapj.sty. Accepted for
publication in The Astrophysical Journal Letter

### Gravitational Collapse in One Dimension

We simulate the evolution of one-dimensional gravitating collisionless
systems from non- equilibrium initial conditions, similar to the conditions
that lead to the formation of dark- matter halos in three dimensions. As in the
case of 3D halo formation we find that initially cold, nearly homogeneous
particle distributions collapse to approach a final equilibrium state with a
universal density profile. At small radii, this attractor exhibits a power-law
behavior in density, {\rho}(x) \propto |x|^(-{\gamma}_crit), {\gamma}_crit
\simeq 0.47, slightly but significantly shallower than the value {\gamma} = 1/2
suggested previously. This state develops from the initial conditions through a
process of phase mixing and violent relaxation. This process preserves the
energy ranks of particles. By warming the initial conditions, we illustrate a
cross-over from this power-law final state to a final state containing a
homogeneous core. We further show that inhomogeneous but cold power-law initial
conditions, with initial exponent {\gamma}_i > {\gamma}_crit, do not evolve
toward the attractor but reach a final state that retains their original
power-law behavior in the interior of the profile, indicating a bifurcation in
the final state as a function of the initial exponent. Our results rely on a
high-fidelity event-driven simulation technique.Comment: 14 Pages, 13 Figures. Submitted to MNRA

### Warp, waves, and wrinkles in the Milky Way

We derive unbiased distance estimates for the Gaia-TGAS data set by correcting for the bias due to the distance dependence of the selection function, which we measure directly from the data. From these distances and proper motions, we estimate the vertical and azimuthal velocities, W and Vϕ, and angular momentum Lz for stars in the Galactic centre and anticentre directions. The resulting mean vertical motion W shows a linear increase with both Vϕ and Lz at 10σ significance. Such a trend is expected from and consistent with the known Galactic warp. This signal extends to stars with guiding centre radii Rg < R0, placing the onset of the warp at R ≲ 7 kpc. At equally high significance, we detect a previously unknown wave-like pattern of W over guiding centre Rg with an amplitude ~1 kms-1 and a wavelength ~2.5 kpc. This pattern is present in both the centre and anticentre directions, consistent with a winding (corrugated) warp or bending wave, likely related to known features in the outer disc (TriAnd and Monoceros overdensities), and may be caused by the interaction with the Sgr dwarf galaxy ~1Gyr ago. The only significant deviation from this simple fit is a stream-like feature near Rg ~9 kpc (|Lz| ~2150 kpc km s-1)

### Models for anisotropic spherical stellar systems with a central point mass and Keplerian velocity dispersions

We add to the lore of spherical, stellar-system models a two-parameter family
with an anisotropic velocity dispersion, and a central point mass (``black
hole''). The ratio of the tangential to radial dispersions, is constant--and
constitutes the first parameter--while each decreases with radius as r^{-1/2}.
The second parameter is the ratio of the central point mass to the total mass.
The Jeans equation is solved to give the density law in closed form: rho\propto
(r/r0)^{-c}/[1+(r/r0)^{3-c}]^2, where r0 is an arbitrary scale factor. The two
parameters enter the density law only through their combination c. At the
suggestion of Tremaine, we also explore models with only the root-sum-square of
the velocities having a Keplerian run, but with a variable anisotropy ratio.
This gives rise to a more versatile class of models, with analytic expressions
for the density law and the dispersion runs, which contain more than one
radius-scale parameter.Comment: 10 pages. Final version to appear in ApJ; minor addition

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