2025ApJ...989L..14R

Owing to sparse spectroscopic observations, the classification of faint satellites as either dark-matter-dominated dwarf galaxies or self-gravitating star clusters remains unresolved. The recently discovered Ursa Major III/UNIONS 1 (UMa3/U1) object, with its measured velocity dispersion, provides a rare observational anchor in this regime. Despite its cluster-like compactness, its inferred dynamical mass-to-light ratio (M_Dyn/L) suggests a dark-matter-dominated nature, prompting interpretations of UMa3/U1 as a microgalaxy, though current measurements remain inconclusive. Thousand-level M_Dyn/L values are not unique to galaxies; self-gravitating dark star clusters (DSCs) can reach comparable levels via energy injection driven by a centrally segregated black hole subsystem (BHSub), which accelerates the evaporation of luminous stars and leads to a supervirial appearance with elevated velocity dispersion. To assess whether UMa3/U1 is a DSC, we conducted direct N-body simulations and identified a model that successfully reproduces both its compact structure and elevated M_Dyn/L, supporting a self-gravitating cluster origin. We find the cluster entered the DSC phase around 4 Gyr ago, with its luminous stars expected to be depleted within the next 1 Gyr, followed by the gradual disruption of the central BHSub over the subsequent Gyr. We broaden our analysis by mapping DSC evolutionary tracks in the size versus total luminosity (L) and M_Dyn/L-L spaces, showing that DSCs occupy a region overlapping with faint, ambiguous satellites. In the M_Dyn/L-L diagram, DSCs trace a transitional channel bridging globular clusters and dwarf galaxies as they rise from M_Dyn/L ≈ 2 to 10⁴M_⊙/L_⊙.