The Formation of Discontinuities in Gas Flows in the ISM and its Relation to the Galactic Synchrotron Radio Emission
Galactic and Intergalactic Magnetic Fields
We show that large-scale motions of the interstellar gas, such as those associated with galactic density waves, easily develop, over a wide range of scales, shocks and discontinuities which are expected to generate turbulence. The latter is supposed to evoke diffusion of magnetic fields and cosmic rays on scales down to a few parsecs. We suggest that these processes may be of major importance in discussions of interconnections between the observed radio emission of the disks of spiral galaxies
... nd the gas density distribution within them. In particular, we predict that the density of cosmic rays and magnetic field energy must be much less contrasted (on scales of ~1 pc and up to the scales of galactic shocks) than the gas density, hence the synchrotron radio emission is not as contrasted as is predicted under the hypothesis of a fully frozen-in magnetic field. The interrelation between gas dynamics and magnetic fields in galactic disks is twofold. On the one hand, magnetic fields and cosmic rays, being frozen into the gas, tend to suppress the development of gas flows characterized by large contrasts of gas density, velocity, etc. On the other hand, the development of gas turbulence strongly enhances the diffusion of magnetic field, thus reducing its influence on gas motion. The gas and field become significantly decoupled, so the gas can develop large contrasts, with the magnetic field (and cosmic rays) only weakly reacting (and acting) on them. One of the main problems here is the origin of the turbulence itself. A widely accepted view is that it is generated by shocks produced by supernovae explosions. We suggest another alternative, according to which the turbulence can be excited in a wide range of scales in gas flows connected with density waves. Here we show that such flows can easily develop shocks and discontinuities which may be regarded as onset of turbulence; on the other hand, these shocks and continuities are expected to represent the source of turbulence under the actual conditions in galactic disks. We have considered an initially circular motion of gas in a disk where a gravitational field of a stellar density wave gradually arises; the 159 R. Beck et al. (eds.), Galactic and Intergalactic Magnetic Fields, 159-162.