Presentation #106.02 in the session Advances in Theory and Numerics for Galactic Dynamics.
Numerical simulations have shown that dynamical friction dissapears inside constant density cores, and that massive objects inside a core can even experience a dynamical buoyancy that ‘pushes’ them outwards. Such phenomena may have important implications for the merger-rate of black holes in the centers of dwarf spheroidals, or for the ability of nuclear star clusters to form via the merging of globular clusters.
Core stalling is not captured by the standard Chandrasekhar treatment of dynamical friction, and even the resonance-picture based on the Lynden-Bell Kalnajs (LBK) torque fails to explain dynamical buoyancy. In this talk I will present two new approaches towards an improved understanding of dynamical friction at large, and of core stalling in particular. After highlighting the shortcomings of the standard formalism based on the LBK torque, I present a more general, self-consistent treatment and demonstrate that it reproduces all aspects of core stalling observed in N-body simulations. Next I present a novel non-perturbative, orbit-based approach that gives valuable insight as to the workings of dynamical friction and dynamical buoyancy. Due to a bifurcation of Lagrange points that occurs when a perturber approaches a core, the near co-rotation resonance orbits that are responsible for friction change their character, which ultimately explains why the direction of the torque acting on a perturber changes sign as it approaches the core region.