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

Following Bi & Davidsen, we perform one-dimensional semianalytic simulations along the lines of sight to model the intergalactic medium (IGM). Since 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 predictions with different observations. For the currently favored low-density cold dark matter model (Omega (m) = 0.4, Omega (Lambda) = 0.6, and h = 0.65), we obtain, using statistics obtained from the transmitted flux, constraints on (1) the combination f = (Omega (B)h(2))(2)/J(-12), where Omega (B) is the baryonic density parameter and J(-12) is the total photoionization rate in units of 10(-12) s(-1), (2) temperature T-o corresponding to the mean density, and (3) the slope gamma of the effective equation of state of the IGM at a mean redshift z similar or equal to 2.5. We find that 0.8 < T-o/(10(4) K) < 2.5 and 1.3 < <gamma> < 2.3, while the constraint obtained on f is 0.020(2) <f < 0.032(2). A reliable lower bound on J(-12) can be used to put a lower bound on <Omega>(B) h(2), which can be compared with similar constraints obtained from big bang nucleosynthesis (BBN) and cosmic microwave background radiation studies. We find that if J(-12) > 1.2, the lower bound on Omega (B)h(2) is in violation of the BBN value.