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

Abstract

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] ∼ -2.1) at a heliocentric distance of 205 ± 20 kpc. The new stellar system has an estimated half-light radius of r<inf>h</inf> = 78<inf>-24</inf>+30 pc and a total luminosity of M<inf>v</inf> ∼ -4.1 ± 0.5 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.