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

Abstract

Following Bi & Davidsen (1997), we perform one dimensional semi analytic 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 favoured LCDM model (Ωm = 0.4, ΩΛ = 0.6 and h = 0.65), we obtain, using statistics obtained from the transmitted flux, constraints on (i) the combination f = (ΩBh2 )2 /J−12, where ΩB is the baryonic density parameter and J−12 is the total photoionisation rate in units of 10−12 s−1 , (ii) temperature T0 corresponding to the mean density and (iii) the slope γ of the effective equation of state of the IGM at a mean redshift z ≃ 2.5. We find that 0.8 < (T0/104 K) < 2.5 and 1.3 < γ < 2.3. while the constraint obtained on f is 0.0202 < f < 0.0322 . A reliable lower bound on J−12 can be used to put a lower bound on ΩBh2 , which can be compared with similar constraints obtained from Big Bang Nucleosynthesis (BBN) and CMBR studies. We find that if J−12 > 1.2, the lower bound on ΩBh2 is in violation of the BBN value.