Distributions of Baryon Fraction on Large Scales in the Universe

The nonlinear evolution of the system consisting of collisional baryons and collisionless dark matter is generally characterized by strong shocks and discontinuities in the baryon fluid. The velocity of baryons will be significantly slowing down at post-shock areas of gravitational strong shocks, which can occur in high overdense as well as low overdense regions. On the other hand, the shocks affect nothing on the dark matter collapsing. Consequently, the baryon fraction would be non-uniform on large scales. We studied these phenomena with simulation samples produced by the WENO hybrid cosmological hydrodynamic/N -body code, which is effective to capture shocks and complex structures with high precision. We find that the baryon fraction in high mass density regions averagely is lower than the cosmic baryon fraction, and many baryons accumulate in the regions with moderate mass density to form a high baryon fraction phase (HBFP) of baryons. In dense regions with ρ/ρ̄ > 100, which are the possible hosts for galaxy clusters, the baryon fraction can be lower than the cosmic baryon fraction by about 10-20% at z ≃ 0. We also find that at z < 2, almost all the HBFP gas locates in the regions with mass density 0.5 < ρ/ρ̄ < 5 and temperature T > 10 K, and conversely, almost all the gas in the areas of 0.5 < ρ/ρ̄ < 5 and with temperature T > 10 K has high baryon fraction. Our simulation samples show that about 3% of the cosmic baryon budget was hidden in the HBFP at redshift z = 3, while this percentage increases to about 14% at the present day. The gas in the HBFP cannot be detected either by Lyα forests of QSO’s absorption spectrum or by soft X-ray background. That is, the HBFP would be missed in the baryon budget given by current observations. Subject headings: cosmology: theory – intergalactic medium – large-scale structure of the universe – methods: numerical – shock waves 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, Nanjing 210008, China