Resonance sweeping by a decelerating Galactic bar

TL;DR

This paper presents evidence that the Galactic bar is decelerating, affecting stellar kinematics and resonance structures, and provides a quantitative estimate of the bar's slowing rate based on local stellar data.

Contribution

It introduces the first quantitative analysis of the Galactic bar's deceleration using stellar kinematics and resonance effects, highlighting the importance of considering slowing patterns in galactic models.

Findings

The Galactic bar is decelerating at a rate of approximately -4.5 km/s/kpc/Gyr.

Deceleration causes resonance sweeping, trapping stars and reproducing observed stellar streams.

Constant pattern speed models are insufficient; deceleration significantly impacts galactic dynamics.

Abstract

We provide the first quantitative evidence for the deceleration of the Galactic bar from local stellar kinematics in agreement with dynamical friction by a typical dark matter halo. The kinematic response of the stellar disk to a decelerating bar is studied using secular perturbation theory and test particle simulations. We show that the velocity distribution at any point in the disk affected by a naturally slowing bar is qualitatively different from that perturbed by a steadily rotating bar with the same current pattern speed $\Omega_{\rm p}$ and amplitude. When the bar slows down, its resonances sweep through phase space, trapping and dragging along a portion of previously free orbits. This enhances occupation on resonances, but also changes the distribution of stars within the resonance. Due to the accumulation of orbits near the boundary of the resonance, the decelerating bar model reproduces with its corotation resonance the offset and strength of the Hercules stream in the local $v_R$-$v_\varphi$ plane and the double-peaked structure of mean $v_R$ in the $L_z$-$\varphi$ plane. At resonances other than the corotation, resonant dragging by a slowing bar is associated with a continuing increase in radial action, leading to multiple resonance ridges in the action plane as identified in the Gaia data. This work shows models using a constant bar pattern speed likely lead to qualitatively wrong conclusions. Most importantly we provide a quantitative estimate of the current slowing rate of the bar $d\Omega_{\rm p}/dt = (-4.5 \pm 1.4)~{\rm km} {\rm s}^{-1} {\rm kpc}^{-1} {\rm Gyr}^{-1}$ with additional systematic uncertainty arising from unmodeled impacts of e.g. spiral arms.