21-cm observations of high-velocity clouds

Extract

(a) Four high-velocity clouds (HVCs) with large negative velocities have been mapped at high sensitivity, with an angular resolution of 12 arcmin and a velocity resolution of 1.8 km s⁻¹ (Cohen & Mirabel, in press). Although these are small isolated clouds they show the same two-component velocity structure that is observed in the large Northern HVC complexes at lower velocity. In addition the cool dense core of one of the clouds is found to be rotating. This rotation, and also the regular velocity patterns within the four clouds, indicates that the clouds are unlikely to be interacting with galactic material. It is interesting that similar observations of the Magellanic Stream do not show narrow velocity components, suggesting that there is a real physical difference between the Stream and other HVCs (Mirabel, Cohen & Davies, in press).