Distributions of the Baryon Fraction on Large Scales in the Universe

(Abridged) The nonlinear evolution of a system consisting of baryons and darkmatter is generally characterized by strong shocks and discontinuities. Thebaryons slow down significantly at postshock areas of gravitational strongshocks, which can occur in high overdense as well as low overdense regions.Consequently, the baryon fraction would be nonuniform on large scales. Westudied these phenomena with simulation samples produced by the WENO hybridcosmological hydrodynamic/N-body code. We find that the baryon fraction in highmass density regions is lower on average than the cosmic baryon fraction, andmany baryons accumulate in the regions with moderate mass density to form ahigh baryon fraction phase (HBFP). In dense regions with rho>100, which are thepossible hosts for galaxy clusters, the baryon fraction can be lower than thecosmic baryon fraction by about 10%--20% at z ~ 0. Our simulation samples showthat about 3% of the cosmic baryon budget was hidden in the HBFP at redshiftz=3, while this percentage increases to about 14% at the present day. The gasin the HBFP cannot be detected either by Ly-alpha forests of QSO absorptionspectra or by soft X-ray background. That is, the HBFP would be missed in thebaryon budget given by current observations.Comment: To appear in ApJ, final versio