Structure formation in disk galaxies via gravitational instability

Disk galaxies abound with intermediate-scale structures in a close geometrical association with spiral arms. Various mechanisms have been proposed as candidates for their origin, but a comprehensive theory should encompass nonlinear effects as well as fundamental physical agents such as self-gravity, magnetic fields, galactic differential rotation, and spiral arms, all of which are known to exist in disks galaxies. Allowing for all these physical elements in this thesis, we use three sets of local, magnetohydrodynamic simulations to investigate the susceptibility of galactic gas disks to the formation of self-gravitating condensations. Our objectives are to understand how self-gravitating modes grow in various environments and thus to assess their consequences for galactic structure formation.; The first set of simulations is for razor-thin, featureless disk models. In such disks, swing amplification that arises under outer galaxy rotation curves is subject to threshold behavior as a consequence of nonlinear effects, with systems of Q < Qc ∼ 1.2--1.4 undergoing gravitational runaway, where the Toomre stability parameter Q is defined by Q ≡ kappa Cs/piGSigma in terms of the epicyclic frequency kappa, the sound speed cs, and a total gas surface density Sigma. Destabilizing, nonlinear secondary processes include parallel fragmentation of filaments, collisions of sheared patches, and rejuvenated swing amplification. Although our finding for Q c is similar to the observed star formation threshold values, the long formation time of clouds makes pure swing amplification an unlikely mechanism for the regulation of galactic star formation. On the other hand, regions of weak shear as in inner galaxies are unstable to magneto-Jeans instability (MJI) in which magnetic tension forces resist the stabilizing Coriolis force, possibly explaining starburst activity in central parts of galaxies.; The second set of simulations is for razor-thin disks with stellar spiral potentials. MJI occurring in shearing and expanding flows off spiral arms develops spur structures in the direction perpendicular to the arms. Spurs are regularly distributed along the arms at an interval about two or three times the local Jeans length at the spiral density peak. In highly nonlinear stages, spurs fragment into dense clumps with masses corresponding to the local Jeans mass at the density peak, possibly evolving into bright arm and interarm H II regions.; The final set of simulations is for three-dimensional, featureless disks. We show that the Parker instability, even combined with self-gravity and galactic shear, is not the main formation mechanism for intermediate-scale structures. By producing in the saturated state mild velocity fluctuations at only 10% of sound speed, the Parker instability is insufficient to generate the observed level of interstellar turbulence as well.; Putting together all the results of our numerical study, we conclude that MJI is at least partly responsible for the formation of intermediate-scale structures in disk galaxies.