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

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 two-dimensional studies (Kim & Ostriker 2002), which showed that a magnetized spiral shock in a thin disk can undergo magnetoJeans instability (MJI), resulting in regularly-spaced interarm spur structures and massive gravitationally-bound fragments. Similar spur (or “feather”) features have recently been seen in high-resolution observations of several galaxies, and massive bound gas condensations are likely the precursors of giant molecular cloud complexes (GMCs) and H II regions. Here, we consider two sets of numerical models: two-dimensional simulations that use a “thick-disk” gravitational kernel, and three-dimensional simulations with explicit vertical stratification. Both models adopt an isothermal equation of state with cs = 7 km s . 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 due to nonzero disk thickness increases the spur spacing to ∼ 10 times the Jeans length at the arm peak, a factor ∼ 3 − 5 times larger than in razor-thin models. Bound clouds that