Numerical simulations of the Lyman alpha forest - a comparison of GADGET-2 and ENZO

We compare simulations of the Lyman alpha forest performed with two different hydrodynamical codes, gadget-2 and ENZO. A comparison of the dark matter power spectrum for simulations run with identical initial conditions show differences of 1-3 per cent at the scales relevant for quantitative studies of the Lyman alpha forest. This allows a meaningful comparison of the effect of the different implementations of the hydrodynamic part of the two codes. Using the same cooling and heating algorithm in both codes, the differences in the temperature and the density probability distribution function are of the order of 10 per cent. The differences are comparable to the effects of box size and resolution on these statistics. When self-converged results for each code are taken into account, the differences in the flux power spectrum - the statistics most widely used for estimating the matter power spectrum and cosmological parameters from Lyman alpha forest data - are about 5 per cent. This is again comparable to the effects of box size and resolution. Numerical uncertainties due to a particular implementation of solving the hydrodynamic or gravitational equations appear therefore to contribute only moderately to the error budget in estimates of the flux power spectrum from numerical simulations. We further find that the differences in the flux power spectrum for ENZO simulations run with and without adaptive mesh refinement are also of the order of 5 per cent or smaller. The latter require 10 times less CPU time making the CPU time requirement similar to that of a version of GADGET-2 that is optimized for Lyman alpha forest simulations.