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

We propose that galactic shocks propagating through interstellar density fluctuations provide a mechanism for the intermittent replenishment, or " pumping, " of the supersonic motions and internal density enhancements observed pervasively within cool atomic and molecular interstellar structures, without necessarily requiring the presence of self-gravity, magnetic fields, or young stars. The shocks are assumed to be due to a variety of galactic sources on a range of scales. An analytic result for the kinematic vorticity generated by a shock passing through a radially-stratified two-dimensional isobaric or isothermal model cloud is derived, assuming that the Mach number is not so large that the cloud is disrupted, and neglecting the shock curvature and cloud distortion. The induced internal velocities are initially a significant fraction of the shock speed divided by the square root of the density contrast, accounting for both the observed linewidth amplitudes and the apparent cloud-to-cloud linewidth-density scaling. The linewidth-size relation could then be interpreted in terms of the well-known power spectrum of a system of uncorrelated shocks. The induced vortical energy should quickly be converted to compressible and MHD modes, and so would be difficult to observe directly, even though it would still be the power source for the other modes. We argue that the shockpump should lead to nested shock-induced structures, providing a cascade mechanism for supersonic " turbulence " and a physical explanation for the fractal-like structure of the cool interstellar medium. The average time between shock exposures for an idealized cloud in our Galaxy is estimated and found to be small enough that the shockpump is capable of sustaining the supersonic motions against readjustment and dissipation, except for the smallest structures. This suggests an explanation of the roughly spatially uniform and nearly sonic linewidths in small coherent cores. We speculate that the avoidance of shock pumping may be necessary for a localized region to form stars, and that the inverse dependence of probability of avoidance on region size may be an important factor in determining the stellar initial mass function. – 2 –