UC San Diego

A theoretical framework is developed to understand 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 interarm region at large pitch angles. 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. In the addition to the formulation, we investigate how 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 response to a hypothesized background density wave supported by the disk stars of the galaxy. In a particular case using galactic parameters at the inner part of M51 galaxy, we show that the normal mode with the maximum growth rate has the wavelength along the spiral arm that matches the observation of spacing of the feathers at around 500 pc. We also demonstrate that the self-gravity is an important parameter governing the feathering instability