Spiral-Induced Secular Evolution of the Gas in Disk Galaxies

Abstract

Stellar spiral arms play a crucial role in the formation and evolution of gaseous structures in disk galaxies by triggering and/or organizing star formation. They also drive secular evolution of both stars and gas and thereby redistribute the mass in galactic disks by exchanging the angular momentum between them. In this thesis, we use both numerical simulations and analytic calculations to investigate the morphological and dynamical evolution of a gaseous disk induced by stellar spiral arms and to explore the relative effects of the associated physical agents such as spiral arms, gaseous self-gravity, and magnetic fields.

Using two-dimensional hydrodynamic simulations, we first investigate nonlinear responses of self-gravitating gas to an imposed stellar spiral potential in galactic disks. By considering various models with different arm strength and pattern speed, we find that the physical properties of imposed spiral potential have profound influences on the shapes and extent of gaseous arms as well as the related mass drift rate. To produce quasi-steady spiral shocks, the gas has to not only move faster than the local sound speed relative to the perturbing potential, but also have sufficient time to respond to one arm before encountering the next arm. From the physical interpretations of our numerical results, we provide a simple expression for the existence of quasi-steady spiral shocks, which is consistent with the previous finding. We also measure the mass drift rates that are in the range of ~0.5-3.0 M⊙/yr inside the corotation radius, and further quantify the relative contribution of shock dissipation (~50%), external torque (~40%), and self-gravitational torque (~10%) to them. The offset between the pitch angles of stellar and gaseous arms is larger for smaller arm strength and larger pattern speed since a deeper potential tends to form shocks closer to the potential minima of the arms. We demonstrate that the distributions of line-of-sight velocities and spiral shock densities can be a diagnostic tool in distinguishing whether the spiral pattern rotates fast or not.

Galactic spiral arms are abundant with interesting gaseous substructures. It has been suggested that arm substructures arise from the wiggle instability (WI) of spiral shocks. While the nature of the WI remained elusive, our recent work without considering magnetic fields shows that the WI is physically originated from the accumulation of potential vorticity generated by deformed shock fronts. To elucidate the characteristics of the WI in more realistic galactic situations, we extend our previous linear stability analysis of spiral shocks by including magnetic fields. We find that magnetic elds reduce the amount of density compression at shocks, making the shock fronts to move toward the upstream direction. Unperturbed magnetic fields stabilize the WI by reducing the density compression factor in the background shocks, rather than perturbed fields derived form the distorted shock fronts. When the spiral-arm forcing is F = 5% of the centrifugal force of galaxy rotation, the maximum growth rate of the WI is found to be about ~0.2-1.5 times the orbital angular frequency for the plasma parameter β=3-∞. The most unstable modes for our fiducial models have a wavelength of ~0.1-0.2 times the arm-to-arm separations, which is well matched with a mean spacing of observed feathers.

Finally, we perform global simulations of galactic gaseous disks by including magnetic fields. The disks have an exponential density distribution and are threaded by azimuthal magnetic fields, but still remain infinitesimally thin. Magnetic fields reduce the shock strength due to the magnetic pressure force, which in turn results in larger offsets between the stellar and gaseous arms and lower mass drift rates than those in the unmagnetized counterparts. We also confirm that the WI is suppressed, although not completely, by the presence of magnetic fields. The measured spacing of the WI-induced nonaxisymmetric features has a range of ~0.1-0.4 times the arm-to-arm distance, consistent with that of observed interarm features. In strongly magnetized models, magnetic fields stretched by the spiral potential accumulate in between two shocks near the corotation radius, developing into magnetic arms with less density but stronger elds than the surrounding regions. This is quite similar to the observed magnetic arms in M83.