Temperature and Entropy Fields of Baryonic Gas in the Universe

(Abridged)The temperature (T) and entropy (S) fields of baryonic gas/IGM areanalyzed using simulation samples by a cosmological hydro/N-body code. We showthat in the nonlinear regime the dynamical similarity between the IGM and darkmatter will be broken by strong shocks. The heating and entropy production bythe shocks breaks the IGM into multiple phases. The mean entropy, or thecosmological entropy floor, is found to be more than 100 h^(-1/3) keV cm^2 inall regions when z < 1. At redshift z ~ 2 - 3, high-entropy gas (S > 50h^(-1/3) keV cm^2) mostly resides in areas on scales larger than 1 h^(-1) Mpcand with density $\rho_{dm}>10^2$. Therefore, gravitational shocks are aneffective preheating mechanism of the IGM, and probably enough to provide theentropy excess of clusters and groups if the epoch of the gas falling incluster cores is not earlier than z ~ 2 - 3. On the other hand, at redshifts z< 4, there is always a more than 90% volume of the low dark matter mass densityregions filled by the IGM with T < 10^4.5 K. Therefore, the multiphasedcharacter and non-Gaussianity of the IGM field would explain thehigh-temperature and high-entropy gas observed in groups and clusters.Comment: 22 pages, 13 figure