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 theintergalactic medium (IGM) and dark matter. Although the evolution of the IGMis dynamically governed by the gravity of the underlying dark matter field,some statistical properties of the IGM inevitably decouple from those of thedark matter once the nonlinearity of the dynamical equations and the stochasticnature of the field is considered. With simulation samples produced by a hybridcosmological hydrodynamic/N-body code, which is effective in capturing shocksand complicated structures with high precision, we find that the one-pointdistributions of the IGM field are systematically different from that of darkmatter as follows: 1.) the one-point distribution of the IGM peculiar velocityfield is exponential at least at redshifts less than 2, while the dark mattervelocity field is close to a Gaussian field; 2.) although the one-pointdistributions of the IGM and dark matter are similar, the point-by-pointcorrelation between the IGM and dark matter density fields significantlydiffers on all scales and redshifts analyzed; 3.) the one-point densitydistributions of the difference between IGM and dark matter fields are highlynon-Gaussian and long tailed. These discrepancies violate the similaritybetween the IGM and dark matter and cannot be explained simply as Jeanssmoothing of the IGM. However, these statistical discrepancies are consistentwith the fluids described by stochastic-force driven nonlinear dynamics.Comment: Accepted in Astrophysical Journal Supplement Serie