3,836 research outputs found
Triaxial Black-Hole Nuclei
We demonstrate that the nuclei of galaxies containing supermassive black
holes can be triaxial in shape. Schwarzschild's method was first used to
construct self-consistent orbital superpositions representing nuclei with axis
ratios of 1:0.79:0.5 and containing a central point mass representing a black
hole. Two different density laws were considered, with power-law slopes of -1
and -2. We constructed two solutions for each power law: one containing only
regular orbits and the other containing both regular and chaotic orbits.
Monte-Carlo realizations of the models were then advanced in time using an
N-body code to verify their stability. All four models were found to retain
their triaxial shapes for many crossing times. The possibility that galactic
nuclei may be triaxial complicates the interpretation of stellar-kinematical
data from the centers of galaxies and may alter the inferred interaction rates
between stars and supermassive black holes.Comment: 4 pages, 4 postscript figures, uses emulateapj.st
Flow Generation by Rotating Colloids in Planar Microchannels
Non-equilibrium structure formation and conversion of spinning to
translational motion of magnetic colloids driven by an external rotating
magnetic field in microchannels is studied by particle-based mesoscale
hydrodynamics simulations. For straight channels, laning is found. In ring
channels, the channel curvature breaks symmetry and leads to a net fluid
transport around the annulus with the same rotational direction as the
colloidal spinning direction. The dependence of the translational velocity on
channel width, ring radius, colloid concentration, and thermal motion is
predicted.Comment: http://epljournal.edpsciences.org/index.php?option=com_article&access=standard&Itemid=129&url=/articles/epl/abs/2010/24/epl13212/epl13212.htm
Chemodynamical history of the Galactic Bulge
The Galactic Bulge can uniquely be studied from large samples of individual
stars, and is therefore of prime importance for understanding the stellar
population structure of bulges in general. Here the observational evidence on
the kinematics, chemical composition, and ages of Bulge stellar populations
based on photometric and spectroscopic data is reviewed. The bulk of Bulge
stars are old and span a metallicity range -1.5<~[Fe/H]<~+0.5. Stellar
populations and chemical properties suggest a star formation timescale below ~2
Gyr. The overall Bulge is barred and follows cylindrical rotation, and the more
metal-rich stars trace a Box/Peanut (B/P) structure. Dynamical models
demonstrate the different spatial and orbital distributions of metal-rich and
metal-poor stars. We discuss current Bulge formation scenarios based on
dynamical, chemical, chemodynamical and cosmological models. Despite impressive
progress we do not yet have a successful fully self-consistent chemodynamical
Bulge model in the cosmological framework, and we will also need more extensive
chrono-chemical-kinematic 3D map of stars to better constrain such models.Comment: 9 figures, 55 pages final version to appear in the Annual Reviews of
Astronomy & Astrophysics, volume 5
Dark Matter Problem in Disk Galaxies
In the generic CDM cosmogony, dark-matter halos emerge too lumpy and
centrally concentrated to host observed galactic disks. Moreover, disks are
predicted to be smaller than those observed. We argue that the resolution of
these problems may lie with a combination of the effects of protogalactic
disks, which would have had a mass comparable to that of the inner dark halo
and be plausibly non-axisymmetric, and of massive galactic winds, which at
early times may have carried off as many baryons as a galaxy now contains. A
host of observational phenomena, from quasar absorption lines and intracluster
gas through the G-dwarf problem point to the existence of such winds. Dynamical
interactions will homogenize and smooth the inner halo, and the observed disk
will be the relic of a massive outflow. The inner halo expanded after absorbing
energy and angular momentum from the ejected material. Observed disks formed at
the very end of the galaxy formation process, after the halo had been reduced
to a minor contributor to the central mass budget and strong radial streaming
of the gas had died down.Comment: 5 pages; submitted to MNRA
Contestable Licensing
We analyze a model of repeated franchise bidding for natural monopoly with contestable licensing - a franchisee halds an (exclusive) license to operate a franchise until another rm offers to pay more for it. In a world where quality is observable but not veri able, the simple regulatory scheme we describe combines market-like incentives with regulatory oversight to generate efficient outcomes.
On the possibility of a warped disc origin of the inclined stellar discs at the Galactic Centre
(Abridged) The Galactic Center (GC) hosts a population of young stars some of
which seem to form mutually inclined discs of clockwise and counter clockwise
rotating stars. We present a warped disc origin scenario for these stars
assuming that an initially flat accretion disc becomes warped due to the
Pringle instability, or due to Bardeen-Petterson effect, before it fragments to
stars. We show that this is plausible if the star formation efficiency
, and the viscosity parameter .
After fragmentation, we model the disc as a collection of concentric, circular,
mutually tilted rings, and construct warped disc models for mass ratios and
other parameters relevant to the GC environment, but also for more massive
discs. We take into account the disc's self-gravity and the torques exerted by
a surrounding star cluster. We show that a self-gravitating low-mass disc () precesses in integrity in the life-time of the stars, but
precesses freely when the torques from a non-spherical cluster are included. An
intermediate-mass disc () breaks into pieces which
precess independently in the self-gravity-only case, and become disrupted in
the presence of the star cluster torques. For a high-mass disc () the evolution is dominated by self-gravity and the disc is broken
but not dissolved. The time-scale after which the disc breaks scales almost
linearly with () for self-gravitating models. Typical values are
longer than the age of the stars for a low mass disc, and are in the range
yr for high and intermediate-mass discs respectively.
None of these models explain the rotation properties of the two GC discs, but a
comparison of them with the clockwise disc shows that the lowest mass model in
a spherical star cluster matches the data best.Comment: 16 pages, 19 figures, abstract abridged to meet arXiv requirements.
Accepted for publication in MNRA
Credits, Crises, and Capital Controls: A Microeconomic Analysis
We analyze the behavior of foreign banks who sequentially provide credit to finance projects in an emerging market. The foreign banks are exposed to both micro-economic risks and the macro-economic risk of a currency crisis, and there are no bailout guarantees. Nevertheless, we show that it is often the case that banks provide too much credit too easily and that this behavior may precipitate the onset of a currency crisis. We demonstrate how the imposition of capital controls in the form of taxes and subsidies on foreign investment may improve the situation. Whereas most of the literature explains currency crises as the consequence of causes that lie within the debtor countries, the general message of our paper may be interpreted as placing part of the blame on the international financial community as well.
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