LARGE-SCALE SHOCKS AND STAR-FORMATION BURSTS IN BARRED GALAXIES

The evolution of large-scale, bar-induced spiral shocks is considered. The main trend of the evolution of concave fronts in the inner parts of the spiral arms, where their curvature is great, is shrinkage. Duri ng this process, the fronts can accelerate, leading to the development of hydrodynamic instability and, as a consequence, of turbulent small scale gas motion behind the shock fronts and to great distortions of the fronts of the waves themselves. The resulting high velocity disper sion in the gas, which also has a high density, creates favorable cond itions for the efficient formation of massive stars. In the case of a common two-arm spiral pattern (Fig. la), these events can give rise to two regions of violent star formation near the center of the bar. An expanding spherical gaseous shell externally bounded by the shock is s ubsequently formed around each region. Two such spherical shock fronts can collide head-on with each other, resulting in two reflected shock s that propagate back to the initial star-forming regions. These refle cted shocks carry along the strongly compressed gas and return it to t he shell centers, thus making possible a recurrent (and, probably, mor e bright) star-formation burst in these two regions. Star-formation bu rsts are actually observed near the centers of the bars in barred spir als. For the other type of spirals (Fig. Ib), the shrinkage of the inn er concave segments occurs near the ends of the bar. The star-forming regions that emerge here can also produce expanding shells with shock fronts around them. In that case, these expanding spherical fronts can interact with new segments of the spiral waves generated by the bar i n the same potential wells (troughs), from which the already shrunk in ner segments have emerged. As regards the weakly curved outer parts of the bar-induced spiral arms, the surfaces of the shock fronts tend to flatten during their evolution. As in the inner parts, new concave se gments of the spiral shock fronts must be formed by the bar in the out er part of the spiral pattern. Thus, a pattern of multifilament spiral arms typical of a number of barred spirals emerges. Possible observat ional tests for this kind of hydrodynamic pattern are discussed.