Efficient adiabatic hydrodynamical simulations of the high-redshift intergalactic medium

We present a post-processing tool for GADGET-2 adiabatic simulations to model various observed properties of the Ly alpha forest at 2.5 <= z <= 4 that enables an efficient parameter estimation. In particular, we model the thermal and ionization histories that are not computed self-consistently by default in GADGET-2. We capture the effect of pressure smoothing by running GADGET-2 at an elevated temperature floor and using an appropriate smoothing kernel. We validate our procedure by comparing different statistics derived from our method with those derived using self-consistent simulations with GADGET-3. These statistics are: line-of-sight density field power spectrum, flux probability distribution function, flux power spectrum, wavelet statistics, curvature statistics, H I column density (N-HI) distribution function, linewidth (b) distribution and b versus log N-HI scatter. For the temperature floor of 10(4) K and typical signal-to-noise ratio of 25, the results agree well within 20 per cent of the self-consistent GADGET-3 simulation. However, this difference is smaller than the expected 1 sigma sample variance for an absorption path length of similar to 5.35 at z = 3. Moreover for a given cosmology, we gain a factor of similar to N in computing time for modelling the intergalactic medium under N >> 1 different thermal histories. In addition, our method allows us to simulate the non-equilibrium evolution of thermal and ionization state of the gas and include heating due to non-standard sources like cosmic rays and high-energy gamma-rays from Blazars.