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

Understand

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).