Swing amplification in stars and gas: Origin of local spirit structure in galaxies

Abstract

We present a study of the growth of local, non-axisymmetric perturbations in gravitationally coupled stars and gas in differentially rotating galactic disk. The amplified perturbations in stars and gas constitute trailing, material spiral features which may be identified with the local spirit features seen in all spiral galaxies. The local, linearized perturbation equations in the sheared frame are derived, and solved to obtain the results for a temporary growth via swing amplification. Due to the lower gas velocity dispersion, the resulting growth in gas is higher, and also the amplified gas features are more tightly wound, as observed. Due to the coupling between stars and gas, the stellar amplification and the range of pitch angles over which it can occur are both increased. Our two-component study can therefore naturally explain the origin of the broad spiral arms in the underlying old stellar populations of galaxies, and the radially increasing arm contrasts -- as observed in the pioneering work by Schweizer (1976), and later by Elmegreen & Elmegreen (1984). The arms are predicted to be broader in gas-rich galaxies, as is indeed observed for example in M33. These new results arise directly due to the inclusion of gas in the problem.