The Galactic Shock Pump: A Source of Supersonic Internal Motions in the Cool Interstellar Medium

We propose that galactic shocks propagating through interstellar densityfluctuations provide a mechanism for the intermittent replenishment, or"pumping," of the supersonic motions and internal density enhancements observedpervasively within cool atomic and molecular interstellar structures, withoutnecessarily requiring the presence of self-gravity, magnetic fields, or youngstars. The shocks are assumed to be due to a variety of galactic sources on arange of scales. An analytic result for the kinematic vorticity generated by ashock passing through a radially-stratified two-dimensional isobaric modelcloud is derived, assuming that the Mach number is not so large that the cloudis disrupted, and neglecting the shock curvature and cloud distortion.Two-dimensional lattic gas hydrodynamic simulations at modest Mach numbers wereused to verify the analytic result. The induced internal velocities areinitially a significant fraction of the shock speed divided by the square rootof the density contrast, accounting for both the observed linewidth amplitudesand the apparent cloud-to-cloud linewidth-density scaling. The linewidth-sizerelation could then be interpreted in terms of the well-known power spectrum ofa system of shocks. The induced vortical energy should quickly be converted tocompressible and MHD modes, and so would be difficult to observe directly, eventhough it would still be the power source for the other modes. The shockpumpthus produces density structure without the necessity of any sort ofinstability. We argue that the shockpump should lead to nested shock-inducedstructures, providing a cascade mechanism for supersonic "turbulence" and aphysical explanation for the fractal-like structure of the cool interstellarmedium.