On the Nature of Ultra-faint Dwarf Galaxy Candidates. III. Horologium I, Pictor I, Grus I, and Phoenix II

Understand

We use deep Gemini/GMOS-S $g,r$ photometry to study the stellar populations of the recently discovered Milky Way satellite candidates Horologium I, Pictor I, Grus I, and Phoenix II.

• Horologium I is most likely an ultra-faint dwarf galaxy at $D_\odot = 68\pm3$ kpc, with $r_h = 23^{+4}_{-3}$pc and $\langle $[Fe/H]$ \rangle = -2.40^{+0.10}_{-0.35}$\,dex.
• It's color-magnitude diagram shows evidence of a split sub-giant branch similar to that seen in some globular clusters.
• Additionally, Gaia DR2 data suggests it is, or was, a member of the Magellanic Cloud group.