Semianalytic Approach to Understanding the Distribution of Neutral Hydrogen in the Universe: Comparison of Simulations with Observations

Following Bi & Davidsen (1997), we perform 1D semi analytic simulations along the lines of sight to model the intergalactic medium (IGM). As this procedure is computationally efficient in probing the parameter space -- and reasonably accurate -- we use it to recover the values of various parameters related to the IGM (for a fixed background cosmology) by comparing the model with various observations. For LCDM model with \Ω_m=0.4, \Ω\Λ=0.6 and h=0.65, we obtain constraints on the baryonic density parameter (\Ω_B), temperature corresponding to the mean density (T_0) and the slope of the equation of state of the IGM (\γ) at a mean redshift z \ 2.5, using statistics obtained from the transmitted flux. Previous attempts in recovering T_0 and \γ made use of the lower cutoff of the distribution of line widths (i.e., b parameter), which can introduce several biases and systematic errors in the inferred values of \γ and T_0. Our method is comparatively free of such errors. We find that 0.8<(T_0/10^4 K)<2.5 and 1.3<\γ<2.3. The constraint obtained on the combination f=(\Ω_B h^2)^2/J₋₁₂ is 0.020^2<f<0.032^2, where J₋₁₂ is the total photoionisation rate in units of 10⁻¹²s⁻¹. We note that a reliable estimate of J₋₁₂ can be used to constrain \Ω_B h^2, and the resulting bound can be compared with similar constraints obtained from Big Bang Nucleosynthesis and CMBR studies.