Part V On the Stability of the Spiral Arms

概要

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The view point that the spiral arms could be in quasi-steady state is suggested from the considerations of instability. The dissipation of the magnetic field in the arms by instability is supposed to compensate the increase in the field due to the wrapping of the magnetic lines of force resulting from the differential rotation in the Galactic disk. These conclusions come from the fact that the possible mechanisms of instability have the growing times of the order of 10^8 year, which is comparable to the characteristic time of the increase of the field. Taking into account the compressibility of fluid the spiral arms are shown to be unstable for the interchange model of gravitational instability against a suitable length of deformation. For instance, its growing time (the e-folding time) is estimated about 10^8 year for the length of perturbation of about 1 kiloparsec along arms. It follows from a rather specific model of the arms that they are also subject to the tearing mode of resistive instability having the growing time of 10^8-10^9 year. A brief account of the intrinsic dissipation inside the arms is presented. Also the derivation of some formulae of gravitational instability and the discussions about the electric conductivity in the interstellar matter are appended.
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