http://arxiv.org/abs/2305.00022
Current theories of dynamical friction on galactic bars are based either on linear perturbation theory, which is valid only in the fast limit where the bar changes its pattern speed rapidly, or on adiabatic theory, which is applicable only in the slow limit where the bar’s pattern speed is near-constant. In this paper, we study dynamical friction on galactic bars spinning down at an arbitrary speed, seamlessly connecting the fast and slow limits. We treat the bar-halo interaction as a restricted $N$-body problem and solve the collisionless Boltzmann equation using the angle-averaged Hamiltonian. The phase-space distribution and density wakes predicted by our averaged model are in excellent agreement with full 3D simulations. In the slow regime where resonant trapping occurs, we show that, in addition to the frictional torque, angular momentum is transferred directly due to the migration of the trapped phase-space: trapped orbits comoving with the resonance typically gain angular momentum, while untrapped orbits leaping over the trapped island lose angular momentum. Due to the negative gradient in the distribution function, gainers typically outnumber the losers, resulting in a net negative torque on the perturber. The torque due to the untrapped orbits was identified by Tremaine & Weinberg, who named the phenomenon dynamical feedback. Here we derive the complete formula for dynamical feedback, accounting for both trapped and untrapped orbits. Using our revised formula, we show that dynamical feedback can comprise up to $30\%$ of the total torque on the Milky Way’s bar.
R. Chiba
Tue, 2 May 23
44/57
Comments: 21 pages, 19 figures, submitted to MNRAS
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