Kinematics of hypervelocity stars in the triaxial halo of the Milky Way

TL;DR

This paper investigates the kinematics of hypervelocity stars in the Milky Way's triaxial halo, proposing a new estimator for dark halo shape based on their position and velocity data at large galactocentric distances.

Contribution

It introduces a method to estimate the Milky Way's dark halo triaxiality using hypervelocity stars' kinematic data, accounting for potential asymmetries in the galaxy's mass distribution.

Findings

Distribution of deflection angles is uniform over 10-180 degrees in a triaxial halo.

Hypervelocity stars' trajectories deviate from radial due to halo asymmetry.

Future surveys can constrain dark matter halo shape using HVS data.

Abstract

Hypervelocity stars (HVSs) ejected by the massive black hole at the Galactic center have unique kinematic properties compared to other halo stars. Their trajectories will deviate from being exactly radial because of the asymmetry of the Milky Way potential produced by the flattened disk and the triaxial dark matter halo, causing a change of angular momentum that can be much larger than the initial small value at injection. We study the kinematics of HVSs and propose an estimator of dark halo triaxiality that is determined only by instantaneous position and velocity vectors of HVSs at large Galactocentric distances (r>~50kpc). We show that, in the case of a substantially triaxial halo, the distribution of deflection angles (the angle between the stellar position and velocity vector) for HVSs on bound orbits is spread uniformly over the range 10--180deg. Future astrometric and deep wide-field surveys should measure the positions and velocities of a significant number of HVSs, and provide useful constraints on the shape of the Galactic dark matter halo.