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 ext ended galactic warm ionized gas layer alters the conventional view of the S DW scenario for star formation: whereas marked density enhancements still o ccur in the midplane, the shock and a prominent high column density structu re 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 sta r 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 res ulting structure resembles a hydraulic jump, or bore, in which gas entering the spiral arm rises suddenly on the upstream side of the arm, then accele rates and angles downward, finally landing on a large downfall region downs tream of the arm.