The vela cloud: A giant H i anomaly in the NGC 3256 group

We present Australia Telescope Compact Array (ATCA) observations of a galaxy-sized intergalactic H I cloud ("the Vela Cloud") in the NGC3256 galaxy group. The group contains the prominent merging galaxy NGC3256, which is surrounded by a number of H I fragments, the tidally disturbed galaxy NGC3263, and several other peculiar galaxies. The Vela Cloud, with an H I mass of 3-5 , resides southeast of NGC3256 and west of NGC3263, within an area of 9′ × 16′ (100 kpc × 175 kpc for an adopted distance of 38Mpc). In our ATCA data the Vela Cloud appears as three diffuse components and contains four density enhancements. The Vela Cloud's properties, together with its group environment, suggest that it has a tidal origin. Each density enhancement contains of H I 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 H I would need to increase by at least a factor of 9 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 H I 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.