The Most Distant Stars in the Milky Way

TL;DR

This paper reports the discovery of the most distant stars in the Milky Way, located over 200 kpc away, using spectroscopic confirmation and color selection, expanding our understanding of the galaxy's outer halo.

Contribution

First spectroscopic confirmation of Milky Way stars beyond 200 kpc, revealing new outer halo giants and providing insights into their possible origins.

Findings

Stars are over 200 kpc away, the most distant known in the Milky Way.

Both stars are moving away from the Galactic center, indicating specific kinematic properties.

Discovery extends the map of the Milky Way's outer halo to previously unexplored regions.

Abstract

We report on the discovery of the most distant Milky Way (MW) stars known to date: ULAS J001535.72$+$015549.6 and ULAS J074417.48$+$253233.0. These stars were selected as M giant candidates based on their infrared and optical colors and lack of proper motions. We spectroscopically confirmed them as outer halo giants using the MMT/Red Channel spectrograph. Both stars have large estimated distances, with ULAS J001535.72$+$015549.6 at $274 \pm 74$ kpc and ULAS J074417.48$+$253233.0 at 238 $\pm$ 64 kpc, making them the first MW stars discovered beyond 200 kpc. ULAS J001535.72$+$015549.6 and ULAS J074417.48$+$253233.0 are both moving away from the Galactic center at $52 \pm 10$ km s$^{-1}$ and $24 \pm 10$ km s$^{-1}$, respectively. Using their distances and kinematics, we considered possible origins such as: tidal stripping from a dwarf galaxy, ejection from the MW's disk, or membership in an undetected dwarf galaxy. These M giants, along with two inner halo giants that were also confirmed during this campaign, are the first to map largely unexplored regions of our Galaxy's outer halo.