Intermittent Features of the Quasar Lyα Transmitted Flux: Results from Cosmological Hydrodynamic Simulations

It has been recently found that the local fluctuations of the QSOs' Lyα absorption spectrum transmitted flux show spiky structures. This implies that the mass fields of the intergalactic medium (IGM) are intermittent. This feature cannot be explained by the clustering evolution of cosmic mass field in the linear regimes and is also difficult to incorporate into the hierarchical clustering scenario. We calculate the structure functions and intermittent exponent of the IGM and H I for full hydrodynamic simulation samples. The result shows the intermittent features of the Lyα transmitted flux fluctuations as well as the mass field of the IGM. We find that within the error bars of current data, all the intermittent behavior of the simulation samples is consistent with the observation. This result is different from our earlier result, which showed that the intermittent behavior of samples generated by pseudohydrodynamic simulation cannot be fitted with observed data. One difference between the pseudohydrodynamic and full hydrodynamic simulations is in treating the dynamical relation between the IGM (or H I) and dark matter fields. The former assumes that the IGM density distribution traces the underlying dark matter point by point on scales larger than the Jeans length in either the linear or nonlinear regime. However, hydrodynamic studies have found that a statistical discrepancy between the IGM field and underlying dark matter in the nonlinear regime is possible. We find that the point-by-point correlation between the IGM density perturbations and dark matter becomes weaker on comoving scales less than 2 h-1 Mpc (in the low-density cold dark matter model), which is larger than the IGM Jeans length.