Resolving the high redshift Lyα 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 Ly alpha forest spectra at 2 <= z <= 5. The mass resolution requirements for the convergence of the mean Ly alpha flux and flux power spectrum at z = 5 are significantly stricter than at lower redshift. This is because transmission in the high redshift Ly alpha 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 Ly alpha forest opacity at z similar or equal to 2-3. We further find that the gas density distribution in our simulations differs significantly from previous results in the literature at large overdensities (Delta > 10). We conclude that studies of the Ly alpha forest at z = 5 using SPH simulations require a gas particle mass of M-gas <= 2 x 10(5) h(-1) M-circle dot, which is greater than or similar to 8 times the value required at z = 2. A box size of at least 40 h(-1) Mpc is preferable at all redshifts.