Orbits Around Black Holes in Triaxial Nuclei
Abstract
We discuss the properties of orbits within the influence sphere of a supermassive black hole (BH), in the case that the surrounding star cluster is nonaxisymmetric. There are four major orbit families; one of these, the pyramid orbits, have the interesting property that they can approach arbitrarily closely to the BH. We derive the orbitaveraged equations of motion and show that in the limit of weak triaxiality, the pyramid orbits are integrable: the motion consists of a twodimensional libration of the major axis of the orbit about the short axis of the triaxial figure, with eccentricity varying as a function of the two orientation angles and reaching unity at the corners. Because pyramid orbits occupy the lowest angular momentum regions of phase space, they compete with collisional loss cone repopulation and with resonant relaxation (RR) in supplying matter to BHs. General relativistic advance of the periapse dominates the precession for sufficiently eccentric orbits, and we show that relativity imposes an upper limit to the eccentricity: roughly the value at which the relativistic precession time is equal to the time for torques to change the angular momentum. We argue that this upper limit to the eccentricity should also apply to evolution driven by RR, with potentially important consequences for the rate of extrememassratio inspirals in lowluminosity galaxies. In giant galaxies, we show that capture of stars on pyramid orbits can dominate the feeding of BHs, at least until such a time as the pyramid orbits are depleted; however this time can be of order a Hubble time.
 Publication:

The Astrophysical Journal
 Pub Date:
 January 2011
 DOI:
 10.1088/0004637X/726/2/61
 arXiv:
 arXiv:1005.0040
 Bibcode:
 2011ApJ...726...61M
 Keywords:

 galaxies: nuclei;
 galaxies: evolution;
 Astrophysics  Astrophysics of Galaxies
 EPrint:
 20 pages, 15 figures