No Observational Evidence for Dark Matter Nor a Large Metallicity Spread in the Extreme Milky Way Satellite Ursa Major III / UNIONS 1
William Cerny, Daisy Bissonette, Alexander P. Ji, Marla Geha, Anirudh Chiti, Simon E. T. Smith, Joshua D. Simon, Andrew B. Pace, Evan N. Kirby, Kim A. Venn, Ting S. Li, Alice M. Luna
TL;DR
The paper challenges the earlier claim that Ursa Major III/UNIONS 1 is a dark-matter-dominated dwarf galaxy by providing deeper Keck/DEIMOS spectroscopy and first metallicity measurements for 12 members. It derives a 95% confidence upper limit on the velocity dispersion of $sigma_v < 2.3$ km s$^{-1}$ and shows a likelihood ratio of about 120:1 for a stellar-only dispersion $sigma_* approx 0.1$ km s$^{-1}$, effectively removing evidence for dark matter. The metallicity analysis yields ⟨[Fe/H]⟩ = -2.65 ± 0.10 (stat) ± 0.30 (zeropoint) with σ_[Fe/H] < 0.35 dex at 95% credibility, suggesting a chemically uniform, very metal-poor population consistent with a star cluster. Collectively, the results are more consistent with UMaIII/U1 being a very low-mass star cluster rather than a DM-dominated dwarf galaxy, though a DM scenario is not completely excluded.
Abstract
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.