Resonance sweeping by a decelerating Galactic bar
  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.
