Differential rotation and turbulence in extended H I disks

When present, extended disks of neutral hydrogen around spiral galaxies sho w a remarkably uniform velocity dispersion of similar to 6 km s(-1). Since stellar winds and supernovae are largely absent in such regions, neither th e magnitude nor the constancy of this number can be accounted for in the cl assical picture in which interstellar turbulence is driven by stellar energ y sources. Here we suggest that magnetic fields with strengths of a few mic rogauss in these extended disks allow energy to be extracted from galactic differential rotation through MHD-driven turbulence. The magnitude and cons tancy of the observed velocity dispersion may be understood if its value is Alfvenic. Moreover, by providing a simple explanation for a lower bound to the gaseous velocity fluctuations, MHD processes may account for the sharp outer edge to star formation in galaxy disks.