Horologium II: a Second Ultra-faint Milky Way Satellite in the Horologium Constellation

Abstract: We report the discovery of a new ultra-faint Milky Way satellite candidate, Horologium II, detected in the Dark Energy Survey Y1A1 public data. Horologium II features a half light radius of $r_{h}=47\pm10$ pc and a total luminosity of $M_{V}=-2.6^{+0.2}_{-0.3}$ that place it in the realm of ultra-faint dwarf galaxies on the size-luminosity plane. The stellar population of the new satellite is consistent with an old ($\sim13.5$ Gyr) and metal-poor ([Fe/H]$\sim-2.1$) isochrone at a distance modulus of $(m-M)=19.46\pm0.20$, or a heliocentric distance of $78\pm8$ kpc, in the color-magnitude diagram. Horologium II has a distance similar to the Sculptor dwarf spheroidal galaxy ($\sim82$ kpc) and the recently reported ultra-faint satellites Eridanus III ($87\pm8$ kpc) and Horologium I ($79\pm8$ kpc). All four satellites are well aligned on the sky, which suggests a possible common origin. As Sculptor is moving on a retrograde orbit within the Vast Polar Structure when compared to the other classical MW satellite galaxies including the Magellanic Clouds, this hypothesis can be tested once proper motion measurements become available.