ATLAS lifts the Cup: Discovery of a New Milky Way satellite in Crater

We announce the discovery of a new Galactic companion found in data from the ESO VST ATLAS survey, and followed up with deep imaging on the 4-m William Herschel Telescope. The satellite is located in the constellation of Crater (the Cup) at a distance of ∼170 kpc. Its half-light radius is rh = 30 pc and its luminosity is MV = −5.5. The bulk of its stellar population is old and metal poor. We would probably have classified the newly discovered satellite as an extended globular cluster were it not for the presence of a handful of blue loop stars and a sparsely populated red clump. The existence of the core helium burning population implies that star formation occurred in Crater perhaps as recently as 400 Myr ago. No globular cluster has ever accomplished the feat of prolonging its star formation by several Gyr. Therefore, if our hypothesis that the blue bright stars in Crater are blue loop giants is correct, the new satellite should be classified as a dwarf galaxy with unusual properties. Note that only 10°to the north of Crater, two ultrafaint galaxies Leo IV and Leo V orbit the Galaxy at approximately the same distance. This hints that all three satellites may once have been closely associated before falling together into the Milky Way halo.