Lognormal semi-numerical simulations of the Lyman-$\hat{I}\pm$ forest: comparison with full hydrodynamic simulations

Abstract

Observations of the Lyman-$\hat{I}\pm$ (Ly$\hat{I}\pm$) 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 semi-numerical 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$\hat{I}\pm$ forest in recovering a set of IGM parameters by comparing with high-resolution Sherwood SPH simulations. We study the recovery of the parameters $T_0$ (temperature of the mean-density IGM), ${\hat{I}}^3$ (slope of the temperature-density relation) and $\hat{I}{“}_{12}$ (hydrogen photoionization rate) at $z\sim 2.5$ using a Markov Chain Monte Carlo (MCMC) technique for parameter estimation. Using three flux statistics, the probability distribution, the mean flux and the power spectrum, values of all three parameters, $T_0$, ${\hat{I}}^3$ and $\hat{I}{“}_{12}$ implied in the SPH simulations are recovered within $1-\ddot{I}ƒ$ ($\sim$ 9, 4 and 1% respectively) of the median (best-fit) values. We verify the validity of our results at different baryon smoothing filter, SNR, box size & resolution, and data seed and confirm that the lognormal model can be used as an efficient tool for modelling the Ly$\hat{I}\pm$ transmitted flux at $z\sim 2.5$.

16 pages, 11 figures; v2: revised version accepted in MNRAS