On the Nature of Ultra-faint Dwarf Galaxy Candidates. III. Horologium I,
  Pictor I, Grus I, and Phoenix II
  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}$\,dex. It's color-magnitude diagram
shows evidence of a split sub-giant branch similar to that seen in some
globular clusters. Additionally, Gaia DR2 data suggests it is, or was, a member
of the Magellanic Cloud group. Pictor I with its compact size ($r_h =
12.9^{+0.3}_{-0.2}$pc) and metal-poor stellar population ($\langle $[Fe/H]$
\rangle = -2.28^{+0.30}_{-0.25}$) closely resembles confirmed star clusters.
Grus I lacks a well-defined centre, but has two stellar concentrations within
the reported half-light radius ($r_h = 1.77^{+0.85}_{-0.39}$ arcmin) and has a
mean metallicity of $\langle $[Fe/H]$ \rangle = -2.5\pm0.3$. Phoenix II has a
half-light radius of $r_h = 12.6\pm2.5$pc and an $\langle $[Fe/H]$ \rangle =
-2.10^{+0.25}_{-0.20}$ and exhibits S-shaped tidal arms extending from its
compact core. Great circles through each of these substructures intersect at
the Large Magellanic Cloud (LMC). This suggests that these objects are, or once
were, satellites of the LMC.
