Is Dark Matter in Spiral Galaxies Cold Gas? II. Fractal Models and Star Non-Formation

Gas cloud models taking into account the recently disclosed fractal structure of cold gas are set up, showing that large errors in the classical gas mass determination based on smooth cloud models can easily follow if the gas is in reality fractal. Fractal clouds must present both optically thin and optically thick clumps in any single wavelength observations. The observed fractal dimension of the cold ISM suggests that mass underestimates by a factor 10 or more are typical. Due to its low temperature (around 3 K), and its condensed fractal structure, together with its low metallicity, the outer gas would be almost invisible for usual detectors. (A&A paper (in press) and figures available by anonymous ftp at obssd8.unige.ch in /pub/fractal as postscript file: dm_paper_II.ps (251k) and FIG*.ps, or papers I & II + figures as a compressed tar file dm_papers.Z.tar (2.1 Mb)).