The motion of massive black holes
Key astrophysical, cosmological and basic physics areas of current research include the growth and evolution of single and binary MBHs and their connections to the evolution of the host galaxy; the rate of supply of single and binary stars to the black hole and their tidal heating, destruction, scattering, capture or ejection; the rate and modes of gravitational wave emission from captured compact objects and from the mergers of binary MBHs; and the emergence of exotic stellar populations around MBHs. The FDP-MBH (Fundamental dynamical processes near massive black holes in galactic nuclei) project used both analytical studies and computer simulations to investigate all these problems. The team made four important discoveries. The slowest mode of randomisation of such systems, by two-body relaxation, has been shown to be an anomalous diffusion process, where 'unlikely' strong scattering events are in fact not so rare. The long-term effects of this process imply that passively evolving galactic nuclei with lower-mass black holes should be dynamically relaxed. A new mode of mass segregation (the tendency of the more massive stars to sink to the centre), named strong mass segregation, has been identified. Here, the massive stars tend to concentrate very sharply at the centre, near the MBH. This increases the rates of strong interactions of stars with the MBH. An 'exotic' fast form of relaxation of angular momentum, resonant relaxation, appears to dominate almost all facets of evolution in the environment near MBHs in galactic nuclei. In particular, it strongly affects the dynamics of small compact objects that fall into MBHs and emit gravitational waves, through the newly found and still not fully understood phenomenon of the Schwarzschild barrier. Resonant relaxation will also interfere with attempts to test basic predictions of general relativity using the orbits of stars around the Galactic MBH, and affect the dynamics of the unusual population of young stars that are seen orbiting around it. Strong effects of resonant stellar torques were found on accretion disks that feed MBHs. These can strongly influence the geometry, properties and variability of accretion disks, and thereby affect the cosmic evolution of both the mass and spin of MBHs.
Keywords
Massive black holes, stars, FDP-MBH, galactic nuclei, resonant relaxation
