Gas dynamics in barred spirals - Gaseous density waves and galactic shocks

Abstract

A strong wave manifestation may be a dominant constituent of the bar structure in barred spiral galaxies : large-scale gaseous density waves and shocks are identified as important phenomena in the gas flow. We generalize the steady-state gas-dynamical studies, previously limited to tightly wound normal spirals, to include barred and open-armed normal spirals. The steady-state response of the gas (non-self-gravitating) to a 5-107 o perturbing potential that is barlike in the inner parts and spiral-like in the outer parts is found to be strong and capable of inducing the formation of large-scale gaseous density waves and shocks in the bar and along the spiral arms. Highly oval streamlines characterize the gas circulation in the inner regions of the disk where large noncircular motions are of the order of 50 km s _1 to 150 km s-1. Strong velocity gradients in the gas flow are particularly pronounced across the bar near the shock. Two types of shocked gas flow are found to be possible in the inner regions of barred spirals. In one type, a shock-focusing phenomenon is discovered which focuses gas in the inner parts outward and gas in the outer parts inward, in the region of convergence where the spiral arm bends from the bar. This shock-focusing phenomenon is thought to account in part for the enhanced star formation activity observed at the ends of the bar structure in many barred spirals. Our analysis constitutes an essential complement to the recent time-evolutionary, numercial hydrodynamical calculations which lack the resolution necessary to compute the detailed structure of the shock and the gas streamlines in the bar region. The present study provides this resolution together with the critical forcing amplitude required to produce offset shocks along the bar. The dark, narrow dust lanes observed along the leading edges of the bar structure in many barred spirals are identified in this study as tracers of such shocks. The interstellar gas-itself a tracer and an important factor influencing the formation of stars and other tracers (e.g., offset dust lanes)-actually provides an outstanding interconnecting link between the overall appearance of a barred spiral and the underlying dynamics of its stellar component. Interpretation of the observed velocity field of one sample barred spiral, NGC 5383, through the application of the theoretical velocity field derived in this steady-state gas flow study indicates that a deeper understanding of barred spirals is obtained.