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We present deep, wide-field Subaru Hyper Suprime-Cam photometry of tworecently discovered satellites of the Milky Way (MW): Columba I and TriangulumII. The color magnitude diagrams of both objects point to exclusively old andmetal-poor stellar populations. We re-derive structural parameters andluminosities of these satellites, and find $M_{\rm V, Col~I} = -4.2\pm0.2$ forCol I and $M_{\rm V, Tri~II} = -1.2\pm0.4$ for Tri II, with correspondinghalf-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 scalingrelations for MW dwarf galaxies. Based on archival data, we derive upper limitson the neutral gas content of these dwarfs, and find that they lack HI, as dothe majority of observed satellites within the MW virial radius. Neithersatellite shows evidence of tidal stripping in the form of extensions ordistortions 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 tidallystripped. Through a suite of orbit simulations, we show that Tri II isapproaching pericenter of its eccentric orbit, a stage at which tidal debris isunlikely to be seen. In addition, we find that Tri II may be on its firstinfall 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 itsstellar mass function, which is similar to those of other MW dwarfs.

Deep Subaru Hyper Suprime-Cam observations of Milky Way satellites Columba I and Triangulum II