Radial average values of the 21 cm velocity dispersion and their standard deviations are shown with filled squares with error bars. The open circles indicate the average energy per square parsec in the same radial bins; energy values have been normalized to 10⁴⁵ ergs pc⁻². The asterisks connected by a dashed line (red) indicate radial averages of the atomic gas surface density in solar masses per square parsec.