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

Effects of galaxy--satellite interactions on bar formation

Aims. We aim to show how encounters with low-mass satellite galaxies may alter the bar formation in a Milky Way-like disc galaxy. Methods. We use high-resolution N-body simulations of a disc galaxy prone to mild bar instability. For realistic initial conditions of satellites, we take advantage of cosmological simulations of Milky Way-like dark matter haloes. Results. The satellites may have a significant impact on the time of bar formation. Some runs with satellites demonstrate a delay, while others show an advancement in bar formation compared to the isolated run, with such time differences reaching $\sim$ 1 Gyr. Meanwhile, the final bar configuration, including its very appearance and the bar characteristics such as the pattern speed and the exponential growth rate of its amplitude are independent of the number of encounters and their orbits. The contribution of satellites with masses below $10^9 M_{\odot}$ is insignificant, unless their pericentre distances are small. We suggest that the encounters act indirectly via inducing perturbations across the disc that evolve to delayed waves in the central part and interfere with an emerging seed bar. The predicted effect for the present-day host galaxy is expected to be even more significant at redshifts $z \gtrsim 0.5$.Comment: 16 pages, 14 figures and 4 table

Outline of the Unified Theory of Spiral and Bar-like Structures in Galaxies

This paper presents a new approach to studying galactic structures. They are considered as the low-frequency normal modes in a disc of orbits precessing at different angular speeds. Such a concept is an adequate alternative to the commonly used approach of treating the disc as a set of individual stars rotating at near-circular orbits around the centre. The problem of determining the normal modes is reduced to a simple integral equation in the form of the classical eigen-value problem, where the eigen-value is directly equal to the pattern speed of the mode, $\Omega_p$. An examination of the general properties of the basic integral equation shows that two types of solutions exist, bar-like and spiral. The numerical solutions of both types are obtained. The characteristic pattern speeds are of the order of the mean orbit precession speed, although for the bar-modes $\Omega_p$ can markedly exceed the maximum precessing speed of orbits. It is shown that the bar-mode grows due to the immediate action of its gravitational field on the stars at the resonance regions. As for the spiral mode, its excitation is probably due to the inner Lindblad resonance that can promote mode growth.Comment: 19 pages, 10 figures, 1 tabl

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 $l \ge 3$. If there is some amount of counter-rotating stars in flattened systems, they generally exhibit the instability independently 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 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 adde