Deep Subaru Hyper Suprime-Cam observations of Milky Way satellites
  Columba I and Triangulum II
  We present deep, wide-field Subaru Hyper Suprime-Cam photometry of two
recently discovered satellites of the Milky Way (MW): Columba I and Triangulum
II. The color magnitude diagrams of both objects point to exclusively old and
metal-poor stellar populations. We re-derive structural parameters and
luminosities of these satellites, and find $M_{\rm V, Col~I} = -4.2\pm0.2$ for
Col I and $M_{\rm V, Tri~II} = -1.2\pm0.4$ for Tri II, with corresponding
half-light radii of $r_{\rm h, Col~I} = 117\pm17$ pc and $r_{\rm h, Tri~II} =
21\pm4$ pc. The properties of both systems are consistent with observed scaling
relations for MW dwarf galaxies. Based on archival data, we derive upper limits
on the neutral gas content of these dwarfs, and find that they lack HI, as do
the majority of observed satellites within the MW virial radius. Neither
satellite shows evidence of tidal stripping in the form of extensions or
distortions in matched-filter stellar density maps or surface density profiles.
However, the smaller Tri II system is relatively metal-rich for its luminosity
(compared to other MW satellites), possibly because it has been tidally
stripped. Through a suite of orbit simulations, we show that Tri II is
approaching pericenter of its eccentric orbit, a stage at which tidal debris is
unlikely to be seen. In addition, we find that Tri II may be on its first
infall into the MW, which helps explain its unique properties among MW dwarfs.
Further evidence that Tri II is likely an ultra-faint dwarf comes from its
stellar mass function, which is similar to those of other MW dwarfs.
