MIXED FRACTAL MODELS FOR THE DISTRIBUTION OF GALAXIES

The ability of fractal models to reproduce the observed distribution o f galaxies is explored. Mixed models, made of fractal density fluctuat ions superposed on a Poisson background, are needed in order to explai n the observed large scale cutoff of the number counts probabilities a nd the power-law behavior of the correlation functions. The distributi on of IRAS galaxies is well fitted by a model in which the Poisson com ponent is five times larger than the fractal component, of dimension 1 .6. A similar mixed fractal model is built for visible galaxies in the Southern Sky Survey, taking into account some dynamic aspects of clus ters. The best fit is obtained for a mixed model in which the Poisson component is as large as the fractal component, of dimension 2. The fi t is however not good at large scales. These results illustrate the co mplexity of the matter distribution in the Universe, and prove the nec essity of considering multifractal models. They also suggest that the multifractallity is confined only to small scales.