Dynamical response of the interstellar medium to explosions at the galactic center

Abstract

The motion of the interstellar medium of a galaxy following an explosion at the galactic center is studied numerically in a simplified geometry under the assumption that the angular momentum of each element of fluid is conserved. The nature of the flow is largely determined by the radial dependence of the circular velocity. When the rotation curve is flat, a single explosion causes a series of shock waves. The expanding shells of dense gas behind the shock fronts are separated by regions of decompressed, inflowing gas. The correlation between density and velocity implies that outflow should be more conspicuous observationally than inflow. For a solid-body rotation curve, multiple shocks do not arise and the density-velocity relation is more complex. In general, outflow should still predominate observationally, but occasional periods of conspicuous inflow are indicated.In veiw of possible applications to our Galaxy, particular attention is directed to the case of constant circular velocity. Owing to the radial variation of the epicyclic frequency for this case, the local length scale of the flow decreases with time everywhere but most rapidly near the center. Consequently, dissipation of radial motions occurs preferentially near the center. The explosion hypothesis for the 3-kpc arm is consistentmore » with the existence of a quiescent nuclear disk if the arm is identified with the expanding ring behind the second shock front. This interpretation is plausible in that the second wave dominates the flow for the longest period of time. Other topics discussed are the emission and absorption profiles of the 21 cm line produced by the expanding rings, departures of observed rotation velocities from circular velocities, and radial motions outside the 3-kpc arm. (AIP)« less