The Lyman α forest in optically thin hydrodynamical simulations

We study the statistics of the Ly alpha forest in a flat Lambda cold dark matter cosmology with the N-body + Eulerian hydrodynamics code NYX. We produce a suite of simulations, covering the observationally relevant redshift range 2 <= z <= 4. We find that a grid resolution of 20 h(-1) kpc is required to produce 1 per cent convergence of Lya forest flux statistics, up to k = 10 h(-1) Mpc. In addition to establishing resolution requirements, we study the effects of missing modes in these simulations, and find that box sizes of L > 40h(-1) Mpc are needed to suppress numerical errors to a sub-per cent level. Our optically thin simulations with the ionizing background prescription of Haardt & Madau reproduce an intergalactic medium density-temperature relation with T-0 approximate to 10(4) K and gamma approximate to 1.55 at z = 2, with a mean transmitted flux close to the observed values. When using the ionizing background prescription of Faucher-Gigu` ere et al., the mean flux is 10-15 per cent below observed values at z = 2, and a factor of 2 too small at z = 4. We show the effects of the common practice of rescaling optical depths to the observed mean flux and how it affects convergence rates. We also investigate the practice of `splicing' results from a number of different simulations to estimate the 1D flux power spectrum and show it is accurate at the 10 per cent level. Finally, we find that collisional heating of the gas from dark matter particles is negligible in modern cosmological simulations.