Constraining Milky Way mass with Hypervelocity Stars

Abstract

Context. Although a variety of techniques have been employed for determining the Milky Way dark matter halo mass distribution, the range of allowed masses spans both light and heavy values. Knowing the precise mass of our Galaxy is important for placing the Milky Way in a cosmological $\Lambda$CDM context. Aims. We show that hypervelocity stars (HVSs) ejected from the center of the Milky Way galaxy can be used to constrain the mass of its dark matter halo. Methods. We use the asymmetry in the radial velocity distribution of halo stars due to escaping HVSs, which depends on the halo potential (escape speed) as long as the round trip orbital time is shorter than the stellar lifetime, to discriminate between different models for the Milky Way gravitational potential. Results. Adopting a characteristic HVS travel time of $330$ Myr, which corresponds to the average mass of main sequence HVSs, we find that current data favors a mass for the Milky Way in the range $(1.2$-$1.9)\times 10^{12} \mathrm{M}_\odot$.