The Stellar Halo in the Inner Milky Way: Predicted Shape and Kinematics

TL;DR

This study uses N-body simulations to explore the shape and kinematics of the old metal-poor stellar halo in the inner Milky Way, revealing its triaxial structure, slow rotation, and limited bar orbit participation.

Contribution

It provides the first detailed simulation-based analysis of the inner halo's shape and kinematics, linking RR Lyrae properties to the Galactic halo's structure.

Findings

Halo density changes from oblate to triaxial

Halo acquires slow rotation consistent with observations

Only 12% of halo orbits follow the bar

Abstract

We have used N-body simulations for the Milky Way to investigate the kinematic and structural properties of the old metal-poor stellar halo in the barred inner region of the Galaxy. We find that the extrapolation of the density distribution for bulge RR Lyrae stars, $\rho\sim r^{-3}$, approximately matches the number density of RR Lyrae in the nearby stellar halo. We follow the evolution of such a tracer population through the formation and evolution of the bar and box/peanut bulge in the N-body model. We find that its density distribution changes from oblate to triaxial, and that it acquires slow rotation in agreement with recent measurements. The maximum radial velocity is $\sim15-25$ km/s at $| l|\!=10^\circ-30^\circ$, and the velocity dispersion is $\sim120$ km/s. Even though the simulated metal-poor halo in the bulge has a barred shape, just $12\%$ of the orbits follow the bar, and it does not trace the peanut/X structure. With these properties, the RR Lyrae population in the Galactic bulge is consistent with being the inward extension of the Galactic metal-poor stellar halo.