A protogalaxy in the local supercluster

The discovery of an isolated H I cloud at a radial velocity of 1275 km/s is reported. At an assumed distance of 20 Mpc, its diameter is about 200 kpc, and its H I mass about 4 x 10 to the 9th solar mass. Although still incompletely mapped, the velocity field exhibits what appears to be ordered rotation: a slowly rising inner rotation curve is followed by a flat portion at Vmax = 30 km/s. The total mass inferred from the rotation curve is about 2.1 x 10 to the 10th solar mass. There is no evidence of a central mass concentration, nor of a stellar population associated with the object. 9 refs