The Discovery of the Faintest Known Milky Way Satellite Using UNIONS

Abstract

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 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.

Microgalaxies in LCDM

“…We discuss our findings in the light of recently discovered Milky Way satellites (Torrealba et al 2019;Mau et al 2020;Smith et al 2024), which have sizes and luminosities at the interface between the globular cluster and dwarf galaxy regime. Could these satellites have their origin in accreted dwarfs and be among the first microgalaxy candidates?…”

“…In particular, we aim to address the nature of recently discovered stellar systems with half-light radii 1  R h /pc  30 and luminosities of 10  L/L e  10 3 (Torrealba et al 2019;Mau et al 2020;Smith et al 2024). These systems populate a parameter space at the boundary between the globular cluster and dwarf galaxy regimes.…”

“…Objects marked as "unidentified" at the boundary of the globular cluster and dwarf galaxy regimes are as compiled in . Data for Ursa Major 3/Unions 1 are from Smith et al (2024).…”

“…(obtained after removing the furthest outlier in velocity as in Smith et al (2024)). Note that this dispersion estimate may still be inflated by the presence of binary stars.…”

“…As long as the Galactocentric velocity of a stellar system can be measured and its orbit constrained, the strength of the tidal field experienced on that orbit can serve to inform whether the presence of dark matter is necessary to avoid full tidal disruption over a time span comparable to the stellar ages in the system. We explore this method with more detail in Errani et al (2023b) for the example of the Milky Way satellite Ursa Major 3/Unions 1 (Smith et al 2024), concluding that only if stabilized by the presence of dark matter could the system survive for more than two radial orbital periods on its current orbit.…”