A Hero's Dark Horse: Discovery of an Ultra-Faint Milky Way Satellite in Pegasus

We report the discovery of an ultra-faint Milky Way satellite galaxy in the constellation of Pegasus. The concentration of stars was detected by applying our overdensity detection algorithm to the SDSS-DR 10 and confirmed with deeper photometry from the Dark Energy Camera at the 4-m Blanco telescope. Fitting model isochrones indicates that this object, Pegasus III, features an old and metal-poor stellar population ([Fe/H]$\sim-2.1$) at a heliocentric distance of $205\pm20$ kpc. The new stellar system has an estimated half-light radius of $r_h=78^{+30}_{-24}$ pc and a total luminosity of $M_{V}\sim-4.1\pm0.5$ that places it into the domain of dwarf galaxies on the size--luminosity plane. Pegasus III is spatially close to the MW satellite Pisces II. It is possible that the two might be physically associated, similar to the Leo IV and Leo V pair. Pegasus III is also well aligned with the Vast Polar Structure, which suggests a possible physical association.