Resolving the high redshift Lyman-α forest in smoothed particle hydrodynamics simulations

We use a large set of cosmological smoothed particle hydrodynamics (SPH) simulations to examine the effect of mass resolution and box size on synthetic Lyα forest spectra at 2 ≤ z ≤ 5. The mass resolution requirements for the convergence of the mean Lyα flux and flux power spectrum at z = 5 are significantly stricter than at lower redshift. This is because transmission in the high redshift Lyα forest is primarily due to underdense regions in the intergalactic medium (IGM), and these are less well resolved compared to the moderately overdense regions which dominate the Lyα forest opacity at z ≃ 2 − 3. We further find that the gas density distribution in our simulations differs significantly from previous results in the literature at large overdensities (∆ > 10). We conclude that studies of the Lyα forest at z = 5 using SPH simulations require a gas particle mass of Mgas ≤ 2 × 10 5 M⊙/h, which is ∼ 8 times the value required at z = 2. A box size of at least 40Mpc/h is preferable at all redshifts.