Lognormal seminumerical simulations of the Lyman α forest: comparison with full hydrodynamic simulations

Abstract

ABSTRACTObservations of the Lyman α (Ly α) forest in spectra of distant quasars enable us to probe the matter power spectrum at relatively small scales. With several upcoming surveys, it is expected that there will be a many-fold increase in the quantity and quality of data, and hence it is important to develop efficient simulations to forward model these data sets. One such seminumerical method is based on the assumption that the baryonic densities in the intergalactic medium (IGM) follow a lognormal distribution. In this work, we test the robustness of the lognormal model of the Ly α forest in recovering a set of IGM parameters by comparing with high-resolution Sherwood smoothed particle hydrodynamics (SPH) simulations. We study the recovery of the parameters T0 (temperature of the mean-density IGM), γ (slope of the temperature–density relation), and Γ12 (hydrogen photoionization rate) at z ∼ 2.5 using a Markov chain Monte Carlo technique for parameter estimation. Using three flux statistics, the probability distribution, the mean flux, and the power spectrum, values of all three parameters, T0, γ, and Γ12, implied in the SPH simulations are recovered within 1 − σ (∼9, 4, and 1 per cent, respectively) of the median (best-fitting) values. We verify the validity of our results at different baryon smoothing filter, signal-to-noise ratio, box size and resolution, and data seed and confirm that the lognormal model can be used as an efficient tool for modelling the Ly α transmitted flux at z ∼ 2.5.