N-body simulation insights into the X-shaped bulge of the Milky Way: Kinematics and distance to the Galactic Centre

TL;DR

This study uses N-body simulations to analyze the Milky Way's X-shaped bulge, revealing its kinematic signatures, estimating the bar angle at 15°, and proposing a geometric method to measure the Galactic Centre's distance with about 5% accuracy.

Contribution

The paper provides detailed simulation-based insights into the kinematics of the Milky Way's X-shaped bulge and introduces a geometric method for accurately determining the Galactic Centre distance.

Findings

Minimum in line-of-sight velocity difference at |b|=4° indicates X-shape signature.

No significant kinematic signature found in azimuthal or vertical velocities.

Distance to Galactic Centre can be measured with ~5% accuracy using X-shape arms.

Abstract

Using simulations of box/peanut- (B/P-) shaped bulges, we explore the nature of the X-shape of the Milky Way's bulge. An X-shape can be associated with a B/P-shaped bulge driven by a bar. By comparing in detail the simulations and the observations we show that the principal kinematic imprint of the X-shape is a minimum in the difference between the near and far side mean line-of-sight velocity along the minor axis. This minimum occurs at around |b| = 4{\deg}, which is close to the lower limit at which the X-shape can be detected. No coherent signature of an X-shape can be found in Galactocentric azimuthal velocities, vertical velocities, or any of the dispersions. After scaling our simulations, we find that a best fit to the BRAVA data leads to a bar angle of 15{\deg}. We also explore a purely geometric method for determining the distance to the Galactic Centre by tracing the arms of the X-shape. We find that we are able to determine this ill-known distance to an accuracy of about 5% per cent with sufficiently accurate distance measurements for the red clump stars in the arms.