Resolving the high redshift Lyman-alpha 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
Lya forest spectra at 2 \leq z \leq 5. The mass resolution requirements for the
convergence of the mean Lya flux and flux power spectrum at z=5 are
significantly stricter than at lower redshift. This is because transmission in
the high redshift Lya 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 Lya 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
(\Delta>10). We conclude that studies of the Lya forest at z=5 using SPH
simulations require a gas particle mass of M_gas \leq 2x10^5 M_sol/h, which is
>8 times the value required at z=2. A box size of at least 40 Mpc/h is
preferable at all redshifts.
