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

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 $\sim 15-25$ km/s at $|l|=10^{\circ }-30^{\circ }$, and the velocity dispersion is $\sim 120$ 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.

arXiv comment5 pages, 5 figures. Accepted for publication in MNRAS Letters

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