Reevaluating UMa3/U1: star cluster or the smallest known galaxy?

ABSTRACT Ursa Major III/UNIONS 1 (UMa3/U1) is the faintest Milky Way satellite discovered to date, exhibiting a half-light radius of $3 \pm 1 \, \text{pc}$ and an absolute V-band magnitude of $+2.2 \pm 0.4$. Previous studies suggest UMa3/U1 is a dwarf galaxy, based on its large internal velocity dispersion and the improbability—indicated by dynamical cluster simulations—of its long-term survival if it were a dark-matter-free star cluster. In this paper, we model the evolution of UMa3/U1 as a star cluster using collisional N-body simulations that include a description of stellar evolution and the external tidal field of the Milky Way, with some simulations including primordial binaries. We find that UMa3/U1 has a substantial remaining lifetime of $2.7 \pm 0.4 \, \text{Gyr}$, primarily due to the retention of compact stellar remnants within the cluster. This retention is facilitated by mass segregation and the preferential loss of low-mass stars. Furthermore, we demonstrate that the observed large velocity dispersion of UMa3/U1 can be successfully reproduced. These results support the possibility that UMa3/U1 is a self-gravitating star cluster. Our simulations reveal that modelling UMa3/U1 as a dark matter–free star cluster produces a markedly altered present-day mass function, driven by a strong depletion of low-mass stars. However, the degree of mass segregation among the visible stars is not statistically significant. We therefore recommend that future observations of UMa3/U1 and other very small Milky Way satellites focus on measuring their present-day mass functions to determine their nature.