Semi analytic approach to understanding the distribution of neutral hydrogen in the universe: Comparison of simulations with observations

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  • 26 December 2000
Abstract
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 \Omega_m=0.4, \Omega_{\Lambda}=0.6 and h=0.65, we obtain constraints on the baryonic density parameter (\Omega_B), temperature corresponding to the mean density (T_0) and the slope of the equation of state of the IGM (\gamma) at a mean redshift z \simeq 2.5, using statistics obtained from the transmitted flux. Previous attempts in recovering T_0 and \gamma 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 \gamma 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<\gamma<2.3. The constraint obtained on the combination f=(\Omega_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^{-12}s^{-1}. We note that a reliable estimate of J_{-12} can be used to constrain \Omega_B h^2, and the resulting bound can be compared with similar constraints obtained from Big Bang Nucleosynthesis and CMBR studies.

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