LINEAR EVOLUTION OF COSMIC BARYONIC MEDIUM ON LARGE SCALES

The evolution of a diffuse baryonic medium in a flat Friedmann universe containing cold or hybrid (cold plus hot) dark matters has been studied in the linear regime. This approximation is reasonable if one considers only the features of the intergalactic medium (IGM) on large scales where the details of energy and mass exchanges between the diffuse medium and lumpy objects (quasars and galaxies) are less important. One can then avoid a direct description of the interaction, but approximate the medium as a polytropic gas. Numerical and analytical calculations showed that for all models considered, the spectrum of density fluctuations of the IGM can be expressed on average as a proportion of those of dark matters by a factor less than 1, in which the only parameter is the Jeans length of the medium. This solution can provide a coherent explanation of cosmological properties of the IGM, including the cosmic Mach number of IGM, Comptonization (Zel'dovich-Sunyaev) parameter, and soft X-ray emission of the IGM. This model showed that the IGM temperature underwent an increase with cosmic time. It implies that (1) clusters may have a halo consisting of hot and thin diffuse baryonic medium, and (2) the temperature of the X-ray gas of clusters increases with the distance from the center of the cluster. The X-ray temperature profiles of Perscus and Virgo Clusters recently given by ROSAT recently appears to consist with these results. The model can also be tested in the following ways: (1) the Comptonization parameter y may not be lower than 10(-4); (2) the IGM X-ray emission may not be larger than that of cosmic X-ray background by a faction of 20% at 0.1 keV and 15% at the 0.3 keV band.