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

It has been recently found that the local fluctuations of the QSO'sLy$\alpha$ absorption spectrum transmitted flux show spiky structures. Thisimplies that the mass fields of the intergalactic medium (IGM) is intermittent.This feature cannot be explained by the clustering evolution of cosmic massfield in the linear regimes and is also difficult to incorporate into thehierarchical clustering scenario. We calculate the structure functions andintermittent exponent of the IGM and HI for full hydrodynamical simulationsamples. The result shows the intermittent features of the Ly$\alpha$transmitted flux fluctuations as well as the mass field of the IGM. We findthat within the error bars of current data, all the intermittent behavior ofthe simulation samples are consistent with the observation. This result isdifferent from our earlier result (Pando et al 2002), which shows that theintermittent behavior of samples generated by pseudo-hydro simulation cannot befitted with observed data. One difference between the pseudo-hydro and fullhydro simulations is in treating the dynamical relation between the IGM (or HI)and dark matter fields. The former assumes that the IGM density distributiontraces the underlying dark matter point-by-point on scales larger than theJeans length in either the linear or nonlinear regimes. However, hydrodynamicstudies have found that a statistical discrepancy between the IGM field andunderlying dark matter in nonlinear regime is possible. We find that thepoint-by-point correlation between the IGM density perturbations and darkmatter become weaker on comoving scales less than 2 h$^{-1}$ Mpc (in LCDMmodel), which is larger than the IGM Jeans length.Comment: AAS Latex file, 38 pages,17 figures included, accepted for publication in Ap