THE VELA CLOUD: A GIANT Hi ANOMALY IN THE NGC 3256 GROUP

摘要

We present Australia Telescope Compact Array (ATCA) observations of a galaxy-sized intergalactic H i 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 H i fragments, the tidally disturbed galaxy NGC 3263, and several otherpeculiar galaxies. The Vela Cloud, with an H I mass of 3-5 x 10~9M, resides southeast of NGC 3256 andwest of NGC 3263, within an area of 9' x 16' (100 kpc x 175 kpc for an adopted distance of 38 Mpc). Inour 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. Eachdensity enhancement contains l08 .M of H I gas, which is sufficient material for the formation of globularcluster progenitors. However, if we represent the enhancements as Bonnor—Ebert spheres, then the pressureof the surrounding H I would need to increase by at least a factor of 9 in order to cause the collapse ofan enhancement. Thus we do not expect them to form massive bound stellar systems like super star clustersor 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.