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 \pm 10 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 ( \sim 13.5 Gyr ) and metal-poor ( [ Fe/H ] \sim - 2.1 ) isochrone at a distance modulus of ( m - M ) = 19.46 \pm 0.20 , or a heliocentric distance of 78 \pm 8 kpc , in the color-magnitude diagram .
Horologium II has a distance similar to the Sculptor dwarf spheroidal galaxy ( \sim 82 kpc ) and the recently reported ultra-faint satellites Eridanus III ( 87 \pm 8 kpc ) and Horologium I ( 79 \pm 8 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 .