ON THE ORIGIN OF SMALL-SCALE STRUCTURE IN SELF-GRAVITATING 2-PHASE GAS

We discuss the origin of multiscale structures in a mixture of a diffu se phase (phase I [PI]) and a dense phase (phase II [PII]) by linear a nalysis, using the notion of multiphase fluid dynamics (MPFD). We can then describe in terms of hydrodynamics how the seeds for small-scale structures, whose scale is smaller than the Jeans length, appear durin g the contraction of clouds. This suggests that our method can describ e the origin of structure in self-gravitating gas systems. It is known that the systems with two-phase mixture of gases generally have two m odes of waves according to MPFD. One mode (f-mode) corresponds to a so und wave, and the other mode (s-mode) is known as a void wave. The s-m ode has smaller phase velocities in the two-phase medium than the f-mo de. And since the s-mode is unstable against the self-gravity, seeds f or smaller scale structure than the ordinary Jeans length can appear. This is because the repulsive effects by ambient phase I gas becomes l ess effective to the gravitational attractions among blobs of phase II gas. So the s-mode coupled with self-gravity may give rise to the clu mpy structure in a molecular cloud, a globular cluster, and the inters tellar medium of a galaxy.