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Spiral Density Wave Shock-induced Star Formation at High Galactic Latitudes

We have modeled the gas response to a spiral density wave (SDW) in a thick, magnetized galactic disk. The inclusion in the model of the vertically extended galactic warm ionized gas layer alters the conventional view of the SDW scenario for star formation: whereas marked density enhancements still occur in the midplane, the shock and a prominent high column density structure extend to high z (the height above the galactic midplane) above the arm. We argue that if the SDW mechanism indeed triggers molecular cloud and star formation, it should do so not only at the midplane but also at distances well above the star-forming thin disk of the conventional picture. The resulting structure resembles a hydraulic jump, or bore, in which gas entering the spiral arm rises suddenly on the upstream side of the arm, then accelerates and angles downward, finally landing on a large downfall region downstream of the arm.