Intercloud structure in a turbulent fractal interstellar medium

Pervasive turbulence and fractal structure in the interstellar gas imply the existence of large holes and gaps, filling ≥ 80% of the volume, which may be identified with the intercloud medium (ICM). Such an ICM needs no supernovae or other localized sources for clearing; extensive supernova clearing seems unlikely anyway on both observational and theoretical grounds. Fractal clouds produce fractal ionization zones (FIZ) in which O-star radiation can travel at least twice as far as in a standard Strömgren sphere, and they contain extensive holes covering ∼50% of the sky through which this radiation can reach the Galactic halo. Clouds in a fractal medium are not randomly distributed like standard clouds in the conventional model; they are highly clumped and clustered. If most of the interstellar gas is in such fractal cloud complexes, then there are on average three clusters per kiloparsec on a line of sight. These three alone produce the observed eight "standard-cloud" absorption lines per kiloparsec by placing about five absorption features on each occupied line of sight through a cloud and none on the unoccupied lines of sight. The mean length of an unoccupied line of sight is ∼600 pc. © 1997. The American Astronomical Society. All rights reserved.