Modeling of a Galactic Spiral Arm as a Bore-Shock Combination

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, 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 a local one, we find that gas passing through it experiences sudden vertical acceleration upstream the arm, giving rise to a shock above the large vertical structure created, midplane density enhancements, and downflow regions behind the jump, as well as, sometimes, secondary jumps. Gravity waves generated in the thick disk appear to promote the formation of the marked density enhancements in the midplane