Testing for Dark Matter in the Outskirts of Globular Clusters
  The proper motions of stars in the outskirts of globular clusters are used to
estimate cluster velocity dispersion profiles as far as possible within their
tidal radii. We use individual color-magnitude diagrams to select high
probability cluster stars for 25 metal-poor globular clusters within 20 kpc of
the sun, 19 of which have substantial numbers of stars at large radii. Of the
19, 11 clusters have a falling velocity dispersion in the 3-6 half mass radii
range, 6 are flat, and 2 plausibly have a rising velocity dispersion. The
profiles are all in the range expected from simulated clusters started at high
redshift in a zoom-in cosmological simulation. The 11 clusters with falling
velocity dispersion profiles are consistent with no dark matter above the
Galactic background. The 6 clusters with approximately flat velocity dispersion
profiles could have local dark matter, but are ambiguous. The 2 clusters with
rising velocity dispersion profiles are consistent with a remnant local dark
matter halo, but need membership confirmation and detailed orbital modeling to
further test these preliminary results.
