2006ApJ...645..861L

Using cosmological hydrodynamic simulations of the ΛCDM model, we present a comparison between the simulation sample and real data sample of H I and He II Lyα transmitted flux in the absorption spectra of the QSO HE 2347-4342. The ΛCDM model is successful in simultaneously explaining the statistical features of both H I and He II Lyα transmitted flux. It includes the following features: (1) The power spectra of the transmitted flux of H I and He II can be well fitted on all scales ≥0.28 h^–1 Mpc for H and ≥1.1 h^–1 Mpc for He. (2) The Doppler parameters of absorption features of He II and H I are found to be turbulent broadening. (3) The ratio of He II to H I optical depths are substantially scattered, due to the significant effect of noise. A large part of the η scatter is due to the noise in the He II flux. However, the real data contain more low-η events than the simulation sample. This discrepancy may indicate that the mechanism leading extra fluctuations on the simulation data, such as a fluctuating UV radiation background, is needed. Yet models of these extra fluctuations should satisfy the following constraints: (1) If the fluctuations are Gaussian, they should be limited by the power spectra of observed H I and He II flux. (2) If the fluctuations are non-Gaussian, they should be limited by the observed non-Gaussian features of the H I and He II flux.