Rimpei Chiba
Theoretical Astrophysicist
Exploring the impact of a decelerating bar on transforming bulge orbits into disc-like orbits
The most metal-poor tail of the Milky Way contains a population of stars with very prograde planar orbits, which is puzzling in both their origin and evolution. We investigated the possibility that the metal poor stars were resonantly dragged by the bar from the inner Galaxy, where many of the old and low-metallicity stars in the Galaxy are located.
The most metal-poor tail of the Milky Way contains a population of stars with very prograde planar orbits, which is puzzling in both their origin and evolution. We investigated the possibility that the metal poor stars were resonantly dragged by the bar from the inner Galaxy, where many of the old and low-metallicity stars in the Galaxy are located.
Origin of reduced dynamical friction by dark matter haloes with net prograde rotation
Dynamical friction has long been known to be suppressed when dark haloes are rotating in the same sense as the disk. We provided a theoretical explanation to this using both linear and nonlinear theory. The key role is played by the direct-radial resonances, which are strongest when orbits are perpendicular to the galactic plane \((L_z = 0)\). Since rotating the halo forward typically generates a positive gradient in the distribution function near \(L_{z} = 0\), these resonances exert a strong positive torque on the bar, weakening the overall dynamical friction.
Dynamical friction has long been known to be suppressed when dark haloes are rotating in the same sense as the disk. We provided a theoretical explanation to this using both linear and nonlinear theory. The key role is played by the direct-radial resonances, which are strongest when orbits are perpendicular to the galactic plane \((L_z = 0)\). Since rotating the halo forward typically generates a positive gradient in the distribution function near \(L_{z} = 0\), these resonances exert a strong positive torque on the bar, weakening the overall dynamical friction.
Dynamical friction and feedback on galactic bars in the general fast-slow regime
We provided a general theory of dynamical friction on galactic bars in the nonlinear (non-perturbative) regime. Our theory seamlessly connects past theories based on the linear approximation, which is only valid in the fast limit, and the extreme adiabatic approximation, which is only valid in the slow limit (i.e. constant pattern speed). In particular, we derived the complete analytical formula for dynamical feedback, a nonlinear process that transfers energy between waves and particles due to the shift in resonances.
We provided a general theory of dynamical friction on galactic bars in the nonlinear (non-perturbative) regime. Our theory seamlessly connects past theories based on the linear approximation, which is only valid in the fast limit, and the extreme adiabatic approximation, which is only valid in the slow limit (i.e. constant pattern speed). In particular, we derived the complete analytical formula for dynamical feedback, a nonlinear process that transfers energy between waves and particles due to the shift in resonances.
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.
