MMT Hypervelocity Star Survey. II. Five New Unbound Stars

TL;DR

This survey discovered five new unbound hypervelocity stars in the Milky Way halo, supporting a Galactic center origin and revealing spatial anisotropy, with implications for understanding the Galaxy's potential and star ejection mechanisms.

Contribution

The paper reports the identification of five new unbound hypervelocity stars, increasing the known sample and providing evidence for their Galactic center origin and anisotropic distribution.

Findings

Identified 5 new unbound hypervelocity stars.

Supports Galactic center as their origin.

Reveals spatial anisotropy of HVSs.

Abstract

We present the discovery of five new unbound hypervelocity stars (HVSs) in the outer Milky Way halo. Using a conservative estimate of Galactic escape velocity, our targeted spectroscopic survey has now identified 16 unbound HVSs as well as a comparable number of HVSs ejected on bound trajectories. A Galactic center origin for the HVSs is supported by their unbound velocities, the observed number of unbound stars, their stellar nature, their ejection time distribution, and their Galactic latitude and longitude distribution. Other proposed origins for the unbound HVSs, such as runaway ejections from the disk or dwarf galaxy tidal debris, cannot be reconciled with the observations. An intriguing result is the spatial anisotropy of HVSs on the sky, which possibly reflects an anisotropic potential in the central 10-100 pc region of the Galaxy. Further progress requires measurement of the spatial distribution of HVSs over the southern sky. Our survey also identifies seven B supergiants associated with known star-forming galaxies; the absence of B supergiants elsewhere in the survey implies there are no new star-forming galaxies in our survey footprint to a depth of 1-2 Mpc.