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 ( 32 ) , which showed that a magnetized spiral shock in a thin disk can undergo magneto-Jeans 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 c _ { s } = 7 km s ^ { -1 } .
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 \sim 10 times the Jeans length at the arm peak , a factor \sim 3 - 5 times larger than in razor-thin models .
Bound clouds that form from spur fragmentation have masses \sim ( 1 - 3 ) \times 10 ^ { 7 } M _ { \odot } each , a factor \sim 3 - 8 times larger than in razor-thin models with the same gas surface density and stellar spiral arm strength .
These condensation masses are comparable to results from other three-dimensional models without spiral structure , and similar to the largest observed GMCs .
The mass-to-flux ratios and specific angular momenta of the bound condensations are lower than large-scale galactic values , as is true for observed GMCs .
We find that unmagnetized or weakly magnetized two-dimensional models are unstable to the “ wiggle instability ” previously identified by Wada & Koda ( 81 ) , and proposed as a potential spur- and clump-forming mechanism .
However , our fully three-dimensional models do not show this effect .
Non-steady motions and strong vertical shear prevent coherent vortical structures from forming , evidently suppressing the wiggle instability that appears in two-dimensional ( isothermal ) simulations .
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 .