Discovery of a Faint Outer Halo Milky Way Star Cluster in the Southern Sky

TL;DR

This paper reports the discovery of a faint, distant star cluster in the Milky Way's outer halo, providing detailed photometric analysis and suggesting it is likely a star cluster with unique properties compared to typical outer halo globular clusters.

Contribution

The discovery and detailed characterization of a new low luminosity outer halo star cluster, Kim 2, including its photometric properties and potential dynamical state.

Findings

Located at ~105 kpc from the Sun

Shares properties with outer halo globular clusters

Likely a star cluster based on dynamical evidence

Abstract

We report the discovery of a new, low luminosity star cluster in the outer halo of the Milky Way. High quality $gr$ photometry is presented, from which a color-magnitude diagram is constructed, and estimates of age, [Fe/H], [$\alpha$/Fe], and distance are derived. The star cluster, which we designate as Kim 2, lies at a heliocentric distance of $\sim105$ kpc. With a half-light radius of $\sim12.8$ pc and ellipticity of $\epsilon\sim0.12$, it shares the properties of outer halo GCs, except for the higher metallicity ([Fe/H]$\sim-1.0$) and lower luminosity ($M_{V}\sim-1.5)$. These parameters are similar to those for the globular cluster AM 4, that is considered to be associated with the Sagittarius dwarf spheroidal galaxy. We find evidence of dynamical mass segregation and the presence of extra-tidal stars that suggests Kim 2 is most likely a star cluster. Spectroscopic observations for radial-velocity membership and chemical abundance measurements are needed to further understand the nature of the object.