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 make the IGM to be in multiple phases. There are no single-valued relations between T or S and the mass densities of the IGM (ρigm) or dark matter (ρdm). The PDFs 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 keV cm in all regions when z ≤ 1. At redshift z ≃ 2− 3, high entropy gas (S > 50 h keV cm) mostly resides in areas on scales larger than 1 h Mpc and with density ρdm > 10 2 (in units of ρdm). Therefore, gravitational shocks are an effective preheating mechanism of the IGM, and 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 more than 90% volume of the low dark matter mass density (ρdm ≤ 2) regions filled by the IGM with temperature less than 10 K. Therefore, the multiple phasedness and non-Gaussianity of the IGM field would be reasonable to account for 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 QSO’s absorption spectrum. Subject headings: cosmology: theory intergalactic medium large-scale structure of the universe methods: numerical National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang, Beijing 100012, China Department of Physics, University of Arizona, Tucson, AZ 85721 Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, 210008