FEATHERING INSTABILITY OF SPIRAL ARMS. I. FORMULATION OF THE PROBLEM

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Detail(s)

Original languageEnglish
Article number45
Journal / PublicationAstrophysical Journal
Volume756
Issue number1
Online published13 Aug 2012
Publication statusPublished - 1 Sep 2012
Externally publishedYes

Abstract

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 interarm 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 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 overdensity contours in the post-shock region are very reminiscent of observed feathering substructures in full magnetohydrodynamic 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 and the OB stars that outline observed grand-design spirals.

Research Area(s)

  • galaxies: ISM, galaxies: structure, instabilities, ISM: kinematics and dynamics, ISM: magnetic fields, magnetohydrodynamics (MHD)