Presentation #310.04D in the session Evolution of Galaxies V.
Gravity plays the key role in structure formation and evolution in astronomical scales. Despite the apparently simple inverse square law of gravitational interactions, the dynamics of many-body systems such as galaxies and dark matter halos is still not sufficiently well understood due to the long range nature of gravity. Moreover, the dynamical evolution or relaxation of galaxies is typically not dominated by two-body encounters but is rather a collective, collisionless process. Based on the timescale of interaction, gravitational encounters can be broadly classified into impulsive, resonant and adiabatic encounters. First, I am going to present a general, non-perturbative formalism to accurately model the impulsive encounters between star-clusters, galaxies or dark matter halos. Then, I shall present a self-consistent, time-dependent, perturbative treatment of resonant relaxation and dynamical friction, which can be used to resolve the long-standing problem of core-stalling observed in N-body simulations: the apparent cessation of dynamical friction-driven in-fall of massive perturbers in spherical host systems with central cores. In our formalism, core-stalling naturally arises from a balance between the retarding torque (dynamical friction) outside and a hitherto unknown enhancing torque (dynamical buoyancy) inside the core-region. To this end, I shall also describe the precise nature of near-resonant field particle orbits responsible for dynamical friction and buoyancy. The dynamical phenomena of core-stalling and buoyancy imply that the observations of offsets of massive objects like globular clusters and black holes from the centers of dark matter dominated dwarf galaxies can potentially constrain their inner dark matter density profile. Finally, I shall apply the above perturbative treatment to a comprehensive analysis of the response of disk galaxies to external perturbers and the resultant stellar oscillations (phase-mixing) and phase-space spirals akin to those observed by GAIA. Impulsive/fast and mildly adiabatic/slow encounters excite different oscillation modes and thus different looking phase-space spirals in the disk. I shall elucidate how these phase-space spirals can be used to constrain the dynamical history and detailed dark matter distribution of our Milky Way galaxy.