The Rotating Bulge and Halo in the Milky Way: Evidence of Angular Momentum Transferred from the Decelerating Bar

Abstract Recent observations indicate that both the Milky Way bulge and inner halo exhibit angular momentum, although the origin and evolution of this prograde signature remain ambiguous. One plausible scenario involves secular evolution induced by the central bar and spiral arms. In this study, we identified a component consisting of 1,175,737 stars with net rotation through the application of a neural network (NN) method. To investigate the composition of this rotating sample and the origin of its rotation, we conducted a test particle simulation incorporating an equilibrium axisymmetric background potential together with a central decelerating bar. The test particles were generated using a distribution function model derived from observational constraints. Our results indicate that the decelerating bar transfers angular momentum to the pseudo-stars, and the rotational profile from our simulation shows strong agreement with observational data. These findings suggest that the rotating sample identified by our NN model is predominantly comprised of bulge, halo, and thick disk stars, and that the central decelerating bar is pivotal in shaping the inner Galaxy’s kinematics through angular momentum transfer.