Spatially Resolved Kinematics of an Ultra-Compact Dwarf Galaxy
  We present the internal kinematics of UCD3, the brightest known ultra-compact
dwarf galaxy (UCD) in the Fornax cluster, making this the first UCD with
spatially resolved spectroscopy. Our study is based on seeing-limited
observations obtained with the ARGUS Integral Field Unit of the VLT/FLAMES
spectrograph under excellent seeing conditions (0.5 - 0.67 arcsec FWHM). The
velocity field of UCD3 shows the signature of weak rotation, comparable to that
found in massive globular clusters. Its velocity dispersion profile is fully
consistent with an isotropic velocity distribution and the assumption that mass
follows the light distribution obtained from Hubble Space Telescope imaging. In
particular, there is no evidence for the presence of an extended dark matter
halo contributing a significant (>~33 per cent within R < 200 pc) mass
fraction, nor for a central black hole more massive than ~5 per cent of the
UCD's mass. While this result does not exclude a galaxian origin for UCD3, we
conclude that its internal kinematics are fully consistent with it being a
massive star cluster.
