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Temperature and Entropy Fields of Baryonic Gas in the Universe

The temperature (T) and entropy (S) fields of baryonic gas, or intergalactic medium (IGM), in the ΛCDM cosmology are analyzed using simulation samples produced by a hybrid cosmological hydrodynamic/N-body code, effective capturing shocks and complex structures with high precision. We show that in the nonlinear regime the dynamical similarity between the IGM and dark matter will be broken in the presence of strong shocks in the IGM. The heating and entropy production by the shocks breaks the IGM into multiple phases. There are no single-value relations between T or S and the mass densities of the IGM (ρigm) or dark matter (ρdm). The probability distribution functions of the temperature and entropy fields are long-tailed, with a power law decreasing on the sides of high temperature or high entropy. These fields are therefore intermittent. The mean entropy, or the cosmological entropy floor, is found to be more than 100 h-1/3 keV cm2 in all regions when z ≤ 1. At redshift z ≃ 2-3, high-entropy gas (S > 50 h-1/3 keV cm2) mostly resides in areas on scales larger than 1 h-1 Mpc and with density ρdm > 102 (in units of dm). Therefore, gravitational shocks are an effective preheating mechanism of the IGM and are probably enough to provide the entropy excess of clusters and groups if the epoch of the gas falling in cluster 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 density (ρdm ≤ 2) regions filled by the IGM with temperature less than 104.5 K. Therefore, the multiphased character and non-Gaussianity of the IGM field would explain the high-temperature and high-entropy gas observed in groups and clusters with low-temperature IGM observed by Lyα forest lines and the intermittency observed by the spikes of the quasi-stellar object's absorption spectrum.