Formation of Spiral-Arm Spurs and Bound Clouds in Vertically Stratified Galactic Gas Disks

Abstract

(Abridged) We investigate the growth of spiral-arm substructure in vertically stratified, self-gravitating, galactic gas disks, using local numerical MHD simulations. Our new models extend our previous 2D studies (Kim & Ostriker 2002), which showed that a magnetized spiral shock in a thin disk can undergo magneto-Jeans instability (MJI), resulting in interarm spur structures and massive fragments. Similar spur features have recently been seen in high-resolution observations of several galaxies. Here, we consider two sets of numerical models: 2D models that use a thick-disk gravitational kernel, and 3D runs with explicit vertical stratification. When disks are sufficiently magnetized and self-gravitating, the result in both sorts of models is the growth of spiral-arm substructure similar to that in our previous razor-thin models. Reduced self-gravity in thick disks increases the spur spacing to ~10 times the Jeans length at the arm peak. Bound clouds that form from spur fragmentation have masses ~(1-3)x10^7 Msun each, a factor ~3-8 times larger than in razor-thin models with the same gas surface density and stellar spiral arm strength. We find that unmagnetized or weakly magnetized 2D models are unstable to the wiggle instability (WI) previously identified by Wada & Koda (2004), and proposed as a potential spur- and clump-forming mechanism. However,our fully 3D models do not show this effect. Non-steady motions and strong vertical shear prevent coherent vortical structures from forming, evidently suppressing the WI that appears in 2D (isothermal) runs. We also find no clear traces of Parker instability in the nonlinear spiral arm substructures that emerge (in self-gravitating models), although conceivably Parker modes may help seed the MJI at early stages since azimuthal wavelengths are similar.