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 magnetohydrodynamic simulations to investigate nonlinear development of the Parker instability, magneto-Jeans instability (MJI), and swing mechanisms in galactic disk models. The disk models are local and 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 Qc ~ 0.75, 0.72, and 0.57 for zero, subequipartition, and equipartition magnetic field cases, respectively. We then pursue fully three-dimensional models. In non-self-gravitating cases, the peak middisk density enhancement from the "pure" Parker instability is less a factor of 2. The dominant growing modes have radial wavelengths λx comparable to the disk scale height H, much shorter than the azimuthal wavelength (λy ~ 10H-20H). 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 ≈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 λy 2πH. Our simulations show that condensations of a local Jeans mass (3 × 107 M☉) grow very rapidly, supporting the idea that MJI is at least partly responsible for the formation of bound cloud complexes in spiral galaxies.
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