Rimpei Chiba
Theoretical Astrophysicist
Dynamical friction and feedback on galactic bars in the general fast-slow regime
When a perturber (e.g. bar, satellite galaxy) changes its frequency in response to dynamical friction, the resonances shift, resulting in an additional transfer of angular momentum: trapped particles dragged by the resonance typically gain angular momentum, while untrapped particles skirting around the trapped phase-space lose angular momentum. This torque, referred to as dynamical feedback, directly depends on the rate of change in pattern speed and therefore modifies the moment of inertia of the perturber. We derived an analytical expression for dynamical feedback and applied the formula to the Galactic bar.
When a perturber (e.g. bar, satellite galaxy) changes its frequency in response to dynamical friction, the resonances shift, resulting in an additional transfer of angular momentum: trapped particles dragged by the resonance typically gain angular momentum, while untrapped particles skirting around the trapped phase-space lose angular momentum. This torque, referred to as dynamical feedback, directly depends on the rate of change in pattern speed and therefore modifies the moment of inertia of the perturber. We derived an analytical expression for dynamical feedback and applied the formula to the Galactic bar.
Oscillating dynamical friction on galactic bars by trapped dark matter
We described the mechanism of dynamical friction in the presence of nonlinear resonant trapping. We showed that, due to the initial gradient in the dark halo's distribution function, net angular momentum is transferred to the halo as trapped particles librate and phase mix. Conventional linear perturbation theory fails to capture this nonlinear behaviour (top). We successfully modeled this process using the resonant angle-action coordinates (bottom) and predicted that dynamical friction on galactic bars may undergo damped oscillation.
We described the mechanism of dynamical friction in the presence of nonlinear resonant trapping. We showed that, due to the initial gradient in the dark halo's distribution function, net angular momentum is transferred to the halo as trapped particles librate and phase mix. Conventional linear perturbation theory fails to capture this nonlinear behaviour (top). We successfully modeled this process using the resonant angle-action coordinates (bottom) and predicted that dynamical friction on galactic bars may undergo damped oscillation.
Resonant angle-action coordinates
Tree-ring structure of Galactic bar resonance
We identified a "tree-ring structure" in the phase space of the bar's resonance: the resonance grows inside-out with stars captured earlier occupying the core of the resonance. We showed that the local stellar metallicity increases monotonically towards the core of the bar's corotation resonance, indicating that the resonance has migrated from the inner galaxy where the metallicity is higher. This observation corroborates the bar's spin-down and hence dynamical friction by dark matter.
We identified a "tree-ring structure" in the phase space of the bar's resonance: the resonance grows inside-out with stars captured earlier occupying the core of the resonance. We showed that the local stellar metallicity increases monotonically towards the core of the bar's corotation resonance, indicating that the resonance has migrated from the inner galaxy where the metallicity is higher. This observation corroborates the bar's spin-down and hence dynamical friction by dark matter.
Resonance sweeping by a decelerating Galactic bar
We provided the first observational implication for the spin-down of the Galactic bar from the velocity distribution of Solar neighborhood stars. In particular, we showed that perturbations by a bar spinning down at a rate consistent with ΛCDM models can naturally explain many of the features in the observational data, including the Hercules stellar stream.
We provided the first observational implication for the spin-down of the Galactic bar from the velocity distribution of Solar neighborhood stars. In particular, we showed that perturbations by a bar spinning down at a rate consistent with ΛCDM models can naturally explain many of the features in the observational data, including the Hercules stellar stream.
