No Observational Evidence for Dark Matter Nor a Large Metallicity Spread in the Extreme Milky Way Satellite Ursa Major III/UNIONS 1

Abstract The extremely low-luminosity, compact Milky Way satellite Ursa Major III/UNIONS 1 (UMaIII/U1; L V = 11 L ⊙ , a 1/2 = 3 pc) was found to have a substantial velocity dispersion at the time of its discovery ( σ v = 3 . 7 − 1.0 + 1.4 km s − 1 ), suggesting that it might be an exceptional, highly dark-matter-dominated dwarf galaxy with very few stars. However, significant questions remained about the system’s dark matter content and nature as a dwarf galaxy, due to the small member sample ( N = 11), possible spectroscopic binaries, and the lack of any metallicity information. Here, we present new spectroscopic observations covering N = 16 members that both dynamically and chemically test the true nature of UMaIII/U1. From higher-precision Keck/DEIMOS spectra, we find a 95% confidence level velocity dispersion limit of σ v < 2.3 km s −1 , with a ∼120:1 likelihood ratio favoring the expected stellar-only dispersion of σ * ≈ 0.1 km s −1 over the original 3.7 km s −1 dispersion. There is now no observational evidence for dark matter in the system. From Keck/LRIS spectra targeting the Ca II K line, we also measure the first metallicities for 12 member stars, finding a mean metallicity of [Fe/H] = − 2.65 ± 0.1 (stat.) ±0.3 (zero-point), with a metallicity dispersion limit of σ [Fe/H] < 0.35 dex (at the 95% credible level). Together, these properties are more consistent with UMaIII/U1 being a star cluster, though the dwarf galaxy scenario is not fully ruled out. Under this interpretation, UMaIII/U1 ranks among the faintest and most metal-poor star clusters yet discovered.