Various instabilities have been proposed as candidates to prompt the condensation of giant , star-forming cloud complexes from the diffuse interstellar medium .
Here , we use three-dimensional ideal MHD simulations to investigate nonlinear development of the Parker , magneto-Jeans ( MJI ) , and swing mechanisms in galactic disk models .
The disk models are local , isothermal , and begin from a vertically-stratified magnetohydrostatic equilibrium state with both gaseous and stellar gravity .
We allow for a range of surface densities and rotational shear profiles , as well as unmagnetized control models .
We first construct axisymmetric equilibria and examine their stability .
Finite disk thickness reduces the critical Toomre stability parameter below unity ; we find Q _ { c } \sim 0.75 , 0.72 , and 0.57 for zero , sub-equipartition , and equipartition magnetic field cases , respectively .
We then pursue fully three-dimensional models .
In non-self-gravitating cases , the peak mid-disk density enhancement from the “ pure ” Parker instability is less a factor of two .
The dominant growing modes have radial wavelengths \lambda _ { x } comparable to the disk scale height H , much shorter than the azimuthal wavelength ( \lambda _ { y } \sim 10 - 20 H ) .
Shearing disks , being more favorable to midplane-symmetric modes , have somewhat different late-time magnetic field profiles from nonshearing disks , but otherwise saturated states are similar .
Late-time velocity fluctuations at 10 % of the sound speed persist , but no characteristic structural signatures of Parker modes remain in the new quasi-static equilibria .
In self-gravitating cases , the development of density structure is qualitatively similar to our previous results from thin-disk simulations .
The Parker instability , although it may help seed structure or tip the balance under marginal conditions , appears to play a secondary role – not affecting , for example , the sizes or spacings of the bound structures that form .
In shearing disks with Q less than a threshold level \approx 1 , swing amplification can produce bound clouds of a few times the local Jeans mass .
The most powerful cloud-condensing mechanism , requiring low-shear conditions as occur in spiral arms or galactic centers , appears to be the MJI .
In thick disks , the MJI occurs for \lambda _ { y } \lower 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } 2 \pi H .
Our simulations show that condensations of a local Jeans mass ( \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 3 \times 10 ^ { 7 } { M _ { \odot } } ) grow very rapidly , supporting the idea that MJI is at least partly responsible for the formation of bound cloud complexes in spiral galaxies .