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

TL;DR

This study demonstrates that a decelerating central bar in the Milky Way transfers angular momentum to stars in the bulge and halo, explaining observed rotation patterns through neural network analysis and simulation.

Contribution

It introduces a neural network method to identify rotating stars and shows that a decelerating bar transfers angular momentum, shaping the Galaxy's inner kinematics.

Findings

Neural network identified over 1.1 million rotating stars.

Simulation shows the bar transfers angular momentum to bulge and halo stars.

Rotational profiles match observational data.

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 (DF) 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 predominantly comprises 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.