The Discovery of the Faintest Known Milky Way Satellite Using UNIONS

We present the discovery of Ursa Major III/UNIONS 1, the least luminous known satellite of the Milky Way, which is estimated to have an absolute V-band magnitude of +2.2(-0.3)(+0.4 )mag, equivalent to a total stellar mass of 16(-5)(+6)M(circle dot). Ursa Major III/UNIONS 1 was uncovered in the deep, wide-field Ultraviolet Near Infrared Optical Northern Survey (UNIONS) and is consistent with an old (tau > 11 Gyr), metal-poor ([Fe/H] similar to -2.2) stellar population at a heliocentric distance of similar to 10 kpc. Despite its being compact (r(h) = 3 +/- 1 pc) and composed of few stars, we confirm the reality of Ursa Major III/UNIONS 1 with Keck II/DEIMOS follow-up spectroscopy and identify 11 radial velocity members, eight of which have full astrometric data from Gaia and are co-moving based on their proper motions. Based on these 11 radial velocity members, we derive an intrinsic velocity dispersion of 3.7(-1.0)(+1.4) km s(-1) but some caveats preclude this value from being interpreted as a direct indicator of the underlying gravitational potential at this time. Primarily, the exclusion of the largest velocity outlier from the member list drops the velocity dispersion to 1.9(-1.1)(+1.4 )km s(-1), and the subsequent removal of an additional outlier star produces an unresolved velocity dispersion. While the presence of binary stars may be inflating the measurement, the possibility of a significant velocity dispersion makes Ursa Major III/UNIONS 1 a high-priority candidate for multi-epoch spectroscopic follow-ups to deduce the true nature of this incredibly faint satellite.