A Dynamical Fossil in the Ursa Minor Dwarf Spheroidal Galaxy

The nearby Ursa Minor dwarf spheroidal (UMi dSph) is one of the most darkmatter dominated galaxies known, with a central mass to light ratio roughlyequal to 70. Somewhat anomalously, it appears to contain morphologicalsubstructure in the form of a second peak in the stellar number density. It isoften argued that this substructure must be transient because it could notsurvive for the > 10 Gyr age of the system, given the crossing time implied byUMi's 8.8 km/s internal velocity dispersion. In this paper, however, we presentevidence that the substructure has a cold kinematical signature, and argue thatUMi's clumpiness could indeed be a primordial artefact. Using numericalsimulations, we demonstrate that substructure is incompatible with the cuspeddark matter haloes predicted by the prevailing Cold Dark Matter (CDM) paradigm,but is consistent with an unbound stellar cluster sloshing back and forthwithin the nearly harmonic potential of a cored dark matter halo. Thus CDMappears to disagree with observation at the least massive, most dark matterdominated end of the galaxy mass spectrum.Comment: Astrophysical Journal (Letters), in pres