The Statistical Discrepancy between the Intergalactic Medium and Dark Matter Fields: One-Point Statistics

We investigate the relationship between the mass and velocity fields of the intergalactic medium (IGM) and dark matter. Although the evolution of the IGM is dynamically governed by the gravity of the underlying dark matter field, some statistical properties of the IGM inevitably decouple from those of the dark matter once the nonlinearity of the dynamical equations and the stochastic nature of the field is considered. With simulation samples produced by a hybrid cosmological hydrodynamic/N-body code, which is effective in capturing shocks and complicated structures with high precision, we find that the one-point distributions of the IGM field are systematically different from that of dark matter as follows: (1) the one-point distribution of the IGM peculiar velocity field is exponential at least at redshifts less than 2, while the dark matter velocity field is close to a Gaussian field; (2) although the one-point distributions of the IGM and dark matter are similar, the point-by-point correlation between the IGM and dark matter density fields significantly differs on all scales and redshifts analyzed; (3) the one-point density distributions of the difference between IGM and dark matter fields are highly non-Gaussian and long tailed. These discrepancies violate the similarity between the IGM and dark matter and cannot be explained simply as Jeans smoothing of the IGM. However, these statistical discrepancies are consistent with the fluids described by stochastic-force-driven nonlinear dynamics.