Identifying galaxy candidates in WSRT HI imaging of ultra-compact high
  velocity clouds
  Ultra-compact high velocity clouds (UCHVCs) were identified in the ALFALFA HI
survey as potential gas-bearing dark matter halos. Here we present higher
resolution neutral hydrogen (HI) observations of twelve UCHVCS with the
Westerbork Synthesis Radio Telescope (WSRT). The UCHVCs were selected based on
a combination of size, isolation, large recessional velocity and high column
density as the best candidate dark matter halos. The WSRT data were tapered to
image the UCHVCs at 210" (comparable to Arecibo) and 105" angular resolution.
In a comparison of the single-dish to interferometer data, we find that the
line flux recovered in the WSRT observations is comparable to that from the
single-dish ALFALFA data. In addition, any structure seen in the ALFALFA data
is reproduced in the WSRT maps at the same angular resolution. At 210'"
resolution all the sources are generally compact with a smooth HI morphology,
as expected from their identification as UCHVCs. At the higher angular
resolution, a majority of the sources break into small clumps contained in a
diffuse envelope. These UCHVCs also have no ordered velocity motion and are
most likely Galactic halo clouds. We identify two UCHVCs, AGC 198606 and AGC
249525, as excellent galaxy candidates based on maintaining a smooth HI
morphology at higher angular resolution and showing ordered velocity motion
consistent with rotation. A third source, AGC 249565, lies between these two
populations in properties and is a possible galaxy candidate. If interpreted as
gas-bearing dark matter halos, the three candidate galaxies have rotation
velocities of 8-15 km/s, HI masses of 0.6-50 x 10^5 Msun, HI radii of 0.3-2
kpc, and dynamical masses of 2-20 x 10^7 Msun for a range of plausible
distances. These are the UCHVCs with the highest column density values in the
ALFALFA HI data and we suggest this is the best way to identify further
candidates.
