A Dynamical Fossil in the Ursa Minor Dwarf Spheroidal Galaxy

The nearby Ursa Minor dwarf spheroidal (UMi dSph) is one of the most dark matter-dominated galaxies known, with a central mass-to-light ratio M / L ~ 70. Somewhat anomalously, it appears to contain morphological substructure in the form of a second peak in the stellar number density. It is often argued that this substructure must be transient, because it could not survive for the ~12 Gyr age of the system, given the crossing time implied by UMi's 8.8 km s -1 internal velocity dispersion. In this Letter, however, we present evidence that the substructure has a cold kinematical signature and argue that UMi's clumpiness could indeed be a primordial artefact. Using numerical simulations, we demonstrate that substructure is incompatible with the cusped dark matter halos predicted by the prevailing cold dark matter (CDM) paradigm but is consistent with an unbound stellar cluster sloshing back and forth within the nearly harmonic potential of a cored dark matter halo. Thus, CDM appears to disagree with observation at the least massive, most dark matter-dominated end of the galaxy mass spectrum.