Spiral Density Waves in M81. II. Hydrodynamic Simulations for Gas Response to Stellar Spiral Density Waves

Gas response to the underlying stellar spirals is explored for M81 using unmagnetized hydrodynamic simulations. Constrained within the uncertainty of observations, 18 simulations are carried out to study the effects of selfgravity and to cover the parameter space comprising three different sound speeds and three different arm strengths. The results are confronted with those data observed at wavelengths of 8 $mu$m and 21 cm. In the outer disk, the ring-like structure observed in 8 $mu$m image is consistent with the response of cold neutral medium with an effective sound speed 7 km s$^{-1}$, while for the inner disk, the presence of spiral shocks can be understood as a result of 4:1 resonances associated with the warm neutral medium with an effective sound speed 19 km s$^{-1}$. Simulations with single effective sound speed alone cannot simultaneously explain the structures in the outer and inner disks. This justifies the coexistence of cold and warm neutral media in M81. The anomalously high streaming motions observed in the northeast arm and the outward shifted turning points in the iso-velocity contours seen along the southwest arm are interpreted as signatures of interactions with companion galaxies. The level of simulated streaming motions narrows down the uncertainty of observed arm strength toward larger amplitudes.

Feathering Instability of Spiral Arms. I: Formulation of the Problem

In this paper we study the feathering substructures along spiral arms by considering the perturbational gas response to a spiral shock. Feathers are density fluctuations that jut out from the spiral arm to the inter-arm region at pitch angles given by the quantum numbers of the doubly-periodic structure. In a localized asymptotic approximation, related to the shearing sheet except that the inhomogeneities occur in space rather than in time, we derive the linearized perturbation equations for a razor-thin disk with turbulent interstellar gas, frozen-in magnetic field, and gaseous self-gravity. Apart from the modal quantum numbers, the individual normal modes of the system depend on seven dimensionless quantities that characterize the underlying time-independent axisymmetric state plus its steady, nonlinear, two-armed spiral-shock (TASS) response to a hypothesized background density-wave supported by the disk stars of the galaxy. We show that some of these normal modes have positive growth rates. Their over-density contours in the post-shock region are very reminiscent of observed feathering substructures in full magnetohydrodynamic (MHD) simulations. The feathering substructures are parasitic instabilities intrinsic to the system; thus, their study not only provides potential diagnostics for important parameters that characterize the interstellar medium of external galaxies, but also yields a deeper understanding of the basic mechanism that drives the formation of the giant molecular clouds (GMCs) and the OB stars that outline observed grand-design spirals.