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In the standard cosmological paradigm, structure formation occurs via gravitational encounters and mergers between galaxies and dark matter halos. These collisionless self-gravitating systems therefore prevail in a state of non-equilibrium or quasi-equilibrium at best. Recent observations show that even our own Milky Way galaxy harbors non-equilibrium features. This calls for a shift of gear from the standard equilibrium galactic dynamics to the less explored non-equilibrium dynamics. This dissertation presents novel theories to describe the relaxation/equilibration of perturbed galaxies (via phase-mixing, Landau damping, etc. ) in the impulsive, adiabatic and near-resonant regimes, as well as the back reaction of the host galaxy response on the perturber, resulting in its secular evolution (dynamical friction). First, we present a non-perturbative treatment of penetrating impulsive encounters between galaxies, a generalization of the standard theory which only works for distant encounters. Next, a general perturbative formalism for the phase-mixing of the response of disk galaxies to external perturbations (e. g. , satellite impacts) is presented, which describes the formation and evolution of non-equilibrium features called phase-spirals akin to those discovered in our Milky Way galaxy by Gaia. Finally, we present a self-consistent perturbative theory and a non-perturbative orbit-based treatment of the near-resonant galaxy response and dynamical friction. These theories, for the first time, explain the origin of dynamical buoyancy and core-stalling, phenomena observed in N-body simulations of the dynamical friction of massive perturbers in cored galaxies that are unexplained in the standard Chandrasekhar and resonance theories. These processes have profound astrophysical implications, e. g. , they can stall and potentially choke supermassive black hole mergers in cored galaxies.
Uddipan Banik (Tue,) studied this question.