On the structure of intermediate- and high-velocity clouds

About 103 early-type stars may be formed in the halo every 107 – 108 yr by the collision of cloudlets within intermediate- and high-velocity clouds at high galactic latitude. Star formation occurs in cloudlets with column densities greater than a few times 1019 cm−2. Energy generated by these stars can accelerate the cloudlets and account for the observed velocity dispersion of 30km s−1. These high-velocity clouds are not virially stable but could be confined by a magnetic field of at least 5 × 10−7 G in the hot coronal gas. Since the cloudlets' random kinetic energy could be renewed by successive bursts of star formation, the clouds, in principle, could be extremely long-lived.