The survival of dynamical fossils in dwarf spheroidal galaxies in
  conventional and modified dynamics
  The survival of unbound density substructure against orbital mixing imposes
strong constraints on the slope of the underlying gravitational potential and
provides a new test on modified gravities. Here we investigate whether the
interpretation that the stellar clump in Ursa Minor (UMi) dwarf spheroidal
galaxy is a `dynamical fossil' is consistent with Modified Newtonian dynamics
(MOND). For UMi mass models inferred by fitting the velocity dispersion
profile, the stellar clump around the second peak of UMi is erased very
rapidly, within 1.25 Gyr (6.5 orbits), even with the inclusion of self-gravity.
We find that the clump can hardly survive for more than 2 Gyr even under more
generous conditions. Alternative scenarios which could lead to a kinematically
cold clump are discussed but, so far, none of them were found to be fully
satisfactory. Our conclusion is that the cold clump in UMi poses a challenge
for both LambdaCDM and MOND.
