Kavli Affiliate: Alexander Ji
| First 5 Authors: William Cerny, William Cerny, , ,

| Summary:
The extremely-low-luminosity, compact Milky Way satellite Ursa Major III / UNIONS 1 (UMaIII/U1; $L_V = 11 L_odot$; $a_1/2 = 3$ pc) was found to have a substantial velocity dispersion at the time of its discovery ($σ_v = 3.7^+1.4_-1.0 rm km s^-1$), 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 UMaIII/U1’s true nature. From higher-precision Keck/DEIMOS spectra, we find a 95% confidence level velocity dispersion limit of $σ_v< 2.3 rm km s^-1$, with a $sim$120:1 likelihood ratio now favoring the expected stellar-only dispersion of $σ_* approx 0.1 rm km s^-1$ over the original $3.7 rm km s^-1$ dispersion. There is now no observational evidence for dark matter in the system. From Keck/LRIS spectra targeting the Calcium II K line, we also measure the first metallicities for 12 member stars, finding a mean metallicity of $rm [Fe/H] = -2.65 ; pm , 0.1$ (stat.) $pm ,0.3$ (zeropoint) with a metallicity dispersion limit of $σ_rm [Fe/H] < 0.35$ 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.
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