The Vela Cloud: A Giant HI Anomaly in the NGC 3256 Group
  We present Australia Telescope Compact Array (ATCA) observations of a
galaxy-sized intergalactic HI cloud (the Vela Cloud) in the NGC 3256 galaxy
group. The group contains the prominent merging galaxy NGC 3256, which is
surrounded by a number of HI fragments, the tidally disturbed galaxy NGC 3263,
and several other peculiar galaxies. The Vela Cloud, with an HI mass of 3-5 *
10**9 solar masses, resides southeast of NGC 3256 and west of NGC 3263, within
an area of 9' x 16' (100 kpc x 175 kpc for an adopted distance of 38 Mpc). In
our ATCA data the Vela Cloud appears as 3 diffuse components and contains 4
density enhancements. The Vela Cloud's properties, together with its group
environment, suggest that it has a tidal origin. Each density enhancement
contains ~10**8 solar masses of HI gas which is sufficient material for the
formation of globular cluster progenitors. However, if we represent the
enhancements as Bonnor-Ebert spheres, then the pressure of the surrounding HI
would need to increase by at least a factor of 6 in order to cause the collapse
of an enhancement. Thus we do not expect them to form massive bound stellar
systems like super star clusters or tidal dwarf galaxies. Since the HI density
enhancements have some properties in common with High Velocity Clouds, we
explore whether they may evolve to be identified with these starless clouds
instead.
