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⊙. 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 (τ>11 Gyr), metal-poor ([Fe/H] ∼−2.2)
stellar population at a heliocentric distance of ∼ 10 kpc. Despite being
compact (rh=3±1 pc) and composed of so 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, 8 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 to true nature of this
incredibly faint satellite.Comment: 21 pages, 9 figures, 3 tables; Accepted for publication in Ap