A Hero's Little Horse: Discovery of a Dissolving Star Cluster in Pegasus

We report the discovery of an ultra-faint stellar system in the constellation of Pegasus. This concentration of stars was detected by applying our overdensity detection algorithm to the SDSS-DR10 and confirmed with deeper photometry from the Dark Energy Camera at the 4-m Blanco telescope. The best-fitting model isochrone indicates that this stellar system, Kim 1, features an old (12 Gyr) and metal-poor ([Fe/H]$\sim-1.7$) stellar population at a heliocentric distance of $19.8\pm0.9$ kpc. We measure a half-light radius of $6.9\pm0.6$ pc using a Plummer profile. The small physical size and the extremely low luminosity are comparable to the faintest known star clusters Segue 3, Koposov 1 & 2, and Mu\~noz 1. However, Kim 1 exhibits a lower star concentration and is lacking a well defined center. It also has an unusually high ellipticity and irregular outer isophotes, which suggests that we are seeing an intermediate mass star cluster being stripped by the Galactic tidal field. An extended search for evidence of an associated stellar stream within the 3 sqr deg DECam field remains inconclusive. The finding of Kim 1 is consistent with current overdensity detection limits and supports the hypothesis that there are still a substantial number of extreme low luminosity star clusters undetected in the wider Milky Way halo.