Feathering Instability of Spiral Arms. II. Parameter Study
  We report the results of a parameter study of the feathering stability in the
galactic spiral arms. A two-dimensional razor-thin magnetized self-gravitating
gas disk with an imposed two-armed stellar spiral structure is considered.
Using the formulation developed previously by Lee and Shu, a linear stability
analysis of the spiral shock is performed in a localized Cartesian geometry.
Results of the parameter study of the base state with a spiral shock are also
presented. The single-mode feathering instability that leads to growing
perturbations may explain the feathering phenomenon found in nearby spiral
galaxies. The self-gravity of the gas, characterized by its average surface
density, is an important parameter which 1) shifts the spiral shock further
downstream; 2) increases the growth rate and decreases the characteristic
spacing of the feathering structure due to the instability. On the other hand,
while the magnetic field suppresses the velocity fluctuation associated with
the feathers, it does not strongly affect their growth rate. Using a set of
typical parameters of the grand-design spiral galaxy M51 at 2 kpc from the
center, the spacing of the feathers with the maximum growth rate is found to be
530 pc, which agrees with the previous observational studies.
