Differential Rotation and Turbulence in Extended HiDisks

When present, extended disks of neutral hydrogen around spiral galaxies showa remarkably uniform velocity dispersion of approx 6 km/s. Since stellar windsand supernovae are largely absent in such regions, neither the magnitude northe constancy of this number can be accounted for in the classical picture inwhich interstellar turbulence is driven by stellar energy sources. Here wesuggest that magnetic fields with strengths of a few microgauss in theseextended disks allow energy to be extracted from galactic differential rotationthrough MHD driven turbulence. The magnitude and constancy of the observedvelocity dispersion may be understood if its value is Alfvenic. Moreover, byproviding a simple explanation for a lower bound to the gaseous velocityfluctuations, MHD processes may account for the sharp outer edge to starformation in galaxy disks.Comment: 9 pages, no figures, to appear in ApJ, LaTeX uses aas2pp4.sty, eliminated duplicate paragrap