Kavli Affiliate: Alexander P. 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 ($sigma_v
= 3.7^+1.4_-1.0 rm km s^-1$), 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 $sigma_v< 2.3 rm km s^-1$, with a $sim$120:1 likelihood ratio now
favoring the expected stellar-only dispersion of $sigma_* approx 0.1 rm km
s^-1$ 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
$sigma_rm [Fe/H] < 0.35$ 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 most metal-poor star clusters yet discovered and is
potentially the first known example of a cluster stabilized by a substantial
population of unseen stellar remnants.
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