86 research outputs found
Effect of angular momentum distribution on gravitational loss-cone instability in stellar clusters around massive BH
Small perturbations in spherical and thin disk stellar clusters surrounding
massive a black hole are studied. Due to the black hole, stars with
sufficiently low angular momentum escape from the system through the loss cone.
We show that stability properties of spherical clusters crucially depend on
whether the distribution of stars is monotonic or non-monotonic in angular
momentum. It turns out that only non-monotonic distributions can be unstable.
At the same time the instability in disk clusters is possible for both types of
distributions.Comment: 14 pages, 7 figures, submitted to MNRA
Gravitational Loss-Cone Instability in Stellar Systems with Retrograde Orbit Precession
We study spherical and disk clusters in a near-Keplerian potential of
galactic centers or massive black holes. In such a potential orbit precession
is commonly retrograde, i.e. direction of the orbit precession is opposite to
the orbital motion. It is assumed that stellar systems consist of nearly radial
orbits. We show that if there is a loss cone at low angular momentum (e.g., due
to consumption of stars by a black hole), an instability similar to loss-cone
instability in plasma may occur. The gravitational loss-cone instability is
expected to enhance black hole feeding rates. For spherical systems, the
instability is possible for the number of spherical harmonics . If
there is some amount of counter-rotating stars in flattened systems, they
generally exhibit the instability independently of azimuthal number . The
results are compared with those obtained recently by Tremaine for distribution
functions monotonically increasing with angular momentum.
The analysis is based on simple characteristic equations describing small
perturbations in a disk or a sphere of stellar orbits highly elongated in
radius. These characteristic equations are derived from the linearized Vlasov
equations (combining the collisionless Boltzmann kinetic equation and the
Poisson equation), using the action-angle variables. We use two techniques for
analyzing the characteristic equations: the first one is based on preliminary
finding of neutral modes, and the second one employs a counterpart of the
plasma Penrose-Nyquist criterion for disk and spherical gravitational systems.Comment: Accepted to Monthly Notices of the Royal Astronomical Society; typos
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Formation Mechanisms for Spirals in Barred Galaxies
We consider a scenario of formation of the spiral structure in barred
galaxies. This scenario includes the new non-resonant mechanism of elongation
of spirals, due to the characteristic behaviour of the gravitational potential
beyond the principal spiral arms
Short Wavelength Analysis of the Evolution of Perturbations in a Two-component Cosmological Fluid
The equations describing a two-component cosmological fluid with linearized
density perturbations are investigated in the small wavelength or large
limit. The equations are formulated to include a baryonic component, as well as
either a hot dark matter (HDM) or cold dark matter (CDM) component. Previous
work done on such a system in static spacetime is extended to reveal some
interesting physical properties, such as the Jeans wavenumber of the mixture,
and resonant mode amplitudes. A WKB technique is then developed to study the
expanding universe equations in detail, and to see whether such physical
properties are also of relevance in this more realistic scenario. The Jeans
wavenumber of the mixture is re-interpreted for the case of an expanding
background spacetime. The various modes are obtained to leading order, and the
amplitudes of the modes are examined in detail to compare to the resonances
observed in the static spacetime results. It is found that some conclusions
made in the literature about static spacetime results cannot be carried over to
an expanding cosmology.Comment: 42 pages, 12 figure
Resolving Gas Dynamics in the Circumnuclear Region of a Disk Galaxy in a Cosmological Simulation
Using a hydrodynamic adaptive mesh refinement code, we simulate the growth
and evolution of a galaxy, which could potentially host a supermassive black
hole, within a cosmological volume. Reaching a dynamical range in excess of 10
million, the simulation follows the evolution of the gas structure from
super-galactic scales all the way down to the outer edge of the accretion disk.
Here, we focus on global instabilities in the self-gravitating, cold,
turbulence-supported, molecular gas disk at the center of the model galaxy,
which provide a natural mechanism for angular momentum transport down to sub-pc
scales. The gas density profile follows a power-law scaling as r^-8/3,
consistent with an analytic description of turbulence in a quasi-stationary
circumnuclear disk. We analyze the properties of the disk which contribute to
the instabilities, and investigate the significance of instability for the
galaxy's evolution and the growth of a supermassive black hole at the center.Comment: 16 pages (includes appendix), submitted to ApJ. Figures here are at
low resolution; for higher resolution version, download
http://casa.colorado.edu/~levinerd/ms.pd
Self-gravitating fragmentation of eccentric accretion disks
We consider the effects of eccentricity on the fragmentation of
gravitationally unstable accretion disks, using numerical hydrodynamics. We
find that eccentricity does not affect the overall stability of the disk
against fragmentation, but significantly alters the manner in which such
fragments accrete gas. Variable tidal forces around an eccentric orbit slow the
accretion process, and suppress the formation of weakly-bound clumps. The
"stellar" mass function resulting from the fragmentation of an eccentric disk
is found to have a significantly higher characteristic mass than that from a
corresponding circular disk. We discuss our results in terms of the disk(s) of
massive stars at ~0.1pc from the Galactic Center, and find that the
fragmentation of an eccentric accretion disk, due to gravitational instability,
is a viable mechanism for the formation of these systems.Comment: 9 pages, 7 figures. Accepted for publication in Ap
On the Possibility of Development of the Explosion Instability in a Two-Component Gravitating System
We obtain an expression for the energy of the density wave propagating in a
multicomponent gravitating medium in the form well known from electrodynamics.
Using the above, the possibility of "triple production" of the quasi-particles,
or waves, with their energies summing up to zero, in a non-equilibrium medium
is demonstrated. That kind of resonance wave interaction is shown to result in
the development of an explosion instability. By the method developed in plasma
physics, the characteristic time of the instability is evaluated.Comment: 15 pages, 3 figures, accepted for publication (JETP
Algebraic damping in the one-dimensional Vlasov equation
We investigate the asymptotic behavior of a perturbation around a spatially
non homogeneous stable stationary state of a one-dimensional Vlasov equation.
Under general hypotheses, after transient exponential Landau damping, a
perturbation evolving according to the linearized Vlasov equation decays
algebraically with the exponent -2 and a well defined frequency. The
theoretical results are successfully tested against numerical -body
simulations, corresponding to the full Vlasov dynamics in the large limit,
in the case of the Hamiltonian mean-field model. For this purpose, we use a
weighted particles code, which allows us to reduce finite size fluctuations and
to observe the asymptotic decay in the -body simulations.Comment: 26 pages, 8 figures; text slightly modified, references added, typos
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