Neutral hydrogen in the Galaxy and the galactic shocks

The effect of the differential rotation on the statistical behaviour of a dynamical system has been studied in the presence of a scalar field. The existence of neutrally stable regimes in a disk-like system has been shown which formally correspond to the Landau levels of a charged particle whose motion is quantized in a magnetic field. The existence also of the so-called anomalous Doppler effect has been pointed out, and a modified form is given to include the differential rotation. Finally, quantitative results are given concerning the instability of the system.