Generation of magnetosonic waves and formation of structures in galaxies

We study the generation of magnetosonic waves in galactic gaseous discs taking account of the magnetic field, differential rotation and self-gravity. The special case of perturbations is considered with the wavevector perpendicular to the magnetic field. The necessary condition of the amplification of seed perturbations is the presence of differential rotation and nonvanishing radial component of the magnetic field that can easily be satisfied in galactic discs. Differential rotation stretches the azimuthal field from the radial one and, therefore, we consider the generation of waves on the time-dependent background magnetic field. Basically, an amplification is rather efficient, and seed perturbations become non-linear already after several rotation periods for a wide range of wavelength. The generated magnetosonic waves can be either non-oscillatory or oscillatory depending on the parameters of gas. If perturbations are Jeans stable, then typically non-oscillatory waves are amplified. However, interplay between self-gravity, magnetic field and rotational shear can change qualitatively the classical Jeans instability, so that the latter becomes oscillatory and tends to be suppressed in galaxies.