The Galactic Shock Pump A Source of Supersonic Internal Motions in the Cool Interstellar Me.pdf
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The Galactic Shock Pump A Source of Supersonic Internal Motions in the Cool Interstellar Me
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The Galactic Shock Pump: A Source of Supersonic Internal
Motions in the Cool Interstellar Medium
Paul Kornreich and John Scalo
Astronomy Department, University of Texas, Austin, TX 78712
The Astrophysical Journal, in Press
Received ; accepted
– 2 –
ABSTRACT
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 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. Two-dimensional lattice gas hydrodynamic simulations at modest Mach numbers
were used to verify the analytic result. 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 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. The shockpump thus produces density structure without
the necessity of any sort of instability. We argue that the shockpump should lead to nested
shock-induced structures, providing a cascade mechanism for supersonic
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