Magnetohydrodynamic Modeling of a Galactic Spiral Arm as a Combination Shock and Hydraulic Jump

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Abstract
We consider the interarm-to-arm transition for gas flow in the Galactic disk, modeled as a thick, magnetized, cloudless layer of gas in hydrostatic equilibrium with external gravity from stars, and having parameters appropriate to the solar neighborhood. We neglect the self-gravity of the gas and shear, and radial variations in gravity. We show that such a transition, if supersonic, must present characteristics of both a hydraulic jump (or bore) and a shock. Our numerical simulations confirm this prediction. Modeling the spiral perturbation as local, we find that flow passing through it builds dense, long-lived vertical structures with high velocity flow over the top, followed by a downstream shock, and sometimes secondary jumps. In addition, gravity waves generated in the thick disk appear to promote the formation of marked density enhancements in the midplane.

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