On the stability of galactic shocks

Abstract

The stability of galactic spiral shocks is considered. A steady-state shock should be checked to see (i) if it is evolutionary; (ii) if its front is stable against bending and torsion; and (iii) if the gas flow far from the front is stable. In the present paper the evolutionary criterion is obtained, which implies that conditions in galaxies may lead to the evolutionary spiral shocks as well as to the nonevolutionary ones. In the latter case a galactic shock cannot persist — it instantly decays, emitting spontaneously spiral waves. This leads to a plausible stratification of the spiral arms, to the formation of the secondary arms, ‘spurs’ and other secondary features. The steady-state gas flow with a galactic shock (Roberts, 1969) turns out to be unstable far from the shock front, the increment being proportional to the velocity gradient. For the spiral shock calculated by Roberts (1969) the instability develops ahead of the shock front with the same growth-time of about 3×107 years for all disturbance scales. This may provide a mechanism to generate turbulence of interstellar gas and to form the patchy structure of spiral arms which are known to include the structural units (gas clouds) on all possible scales.