Diffuse H in a Fractal Interstellar Medium

Correlated Motions and Fractal Structures

Interstellar gas is hierarchically clumped, with condensations and filaments inside larger, more tenuous condensations and filaments (Scalo 1985). This seems to be the case for all irregular structures that are not obvious shells or other disturbances (although even some shells are nested inside bigger shells). If we consider that the regular structures, like shells, spiral density waves, whole galaxy disks, etc., are shaped directly by organized forces and respond on specific scales to boundary or initial conditions, then most of the interstellar structure that is far removed from such boundary forces is hierarchically clumped. This includes the low density (undisturbed) gas between shells, the gas that lies inside the shell periphery, i.e., the swept-up gas, the gas inside spiral arms (as opposed to the arms themselves), and everywhere else that is free of confinement.

The origin of this pervasive hierarchical structure seems to be turbulence. Turbulence is characterized by correlated motions in which regions close to each other comove in space, and regions further apart move more independently (Larson 1981). Turbulence also has the property that there is no characteristic scale in many problems, aside from the molecular collision mean free path and the scale of the whole system, where the walls or other confinement structures occur. Scale-free structures are fractal (Mandelbrot 1983), so turbulent gas can become fractal far from its boundaries on a time scale comparable to the crossing time.

© Copyright Astronomical Society of Australia 1997