Three-Dimensional Simulations of Parker, Magneto-Jeans, and Swing Instabilities in Shearing Galactic Gas Disks

Abstract

We use 3D MHD simulations to investigate nonlinear development of the Parker, magneto-Jeans (MJI), and swing mechanisms in galactic disks. The model disks are local, isothermal, and begin from a vertically-stratified equilibrium. We first construct axisymmetric equilibria and examine their stability. Finite disk thickness reduces the critical Toomre Q parameter below unity; we find Q_c \~ 0.75, 0.72, and 0.57 for \beta=\infty, 10, and 1 cases, respectively. We then pursue fully 3D models. In non-self-gravitating cases, the peak density enhancement from the `pure' Parker instability is less a factor of two. The dominant growing modes have radial wavelengths comparable to the disk scale height H, much shorter than the azimuthal wavelength (~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 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. 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 appears to be the MJI. Our simulations show that condensations of a local Jeans mass (~3\times 10^7 M_sun) grow very rapidly, supporting the idea that MJI is at least partly responsible for the formation of bound cloud complexes in spiral galaxies.