Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007mnras.379..573p&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Volume 379, Issue 2, pp. 573-596.
Astronomy and Astrophysics
Astronomy
12
Galaxy: Centre, Galaxies: Kinematics And Dynamics
Scientific paper
We study spherical and disc clusters in a near-Keplerian potential of galactic centres or massive black holes. In such a potential orbit precession is commonly retrograde, that is, the 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 l >= 3. If there is some amount of counter-rotating stars in flattened systems, they generally exhibit the instability independent of azimuthal number m. 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 disc 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 analysing 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 disc and spherical gravitational systems.
Polyachenko Evgenii V.
Polyachenko Valerij L.
Shukhman Ilia G.
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