Convergence of small scale Ly structure at high-z under different reionization scenarios

The Ly alpha forest (LAF) at z > 5 probes the thermal and reionization history of the intergalactic medium (IGM) and the nature of dark matter, but its interpretation requires comparison to cosmological hydrodynamical simulations. At high-z, convergence of these simulations is more exacting since transmission is dominated by underdense voids that are challenging to resolve. With evidence mounting for a late end to reionization, small structures down to the sub-kpc level may survive to later times than conventionally thought due to the reduced time for pressure smoothing to impact the gas, further tightening simulation resolution requirements. We perform a suite of simulations using the Eulerian cosmological hydrodynamics code Nyx, spanning domain sizes of 1.25 - 10h(-1)Mpc and 5 - 80 h(-1)kpc cells, and explore the interaction of these variables with the timing of reionization on the properties of the matter distribution and the simulated LAF at z = 5.5. In observable Ly alpha power, convergence within 10 percent is achieved for k < 0.1skm(-1), but larger k shows deviation of up to 20 percent. While a later reionization retains more small structure in the density field, because of the greater thermal broadening there is little difference in the convergence of LAF power between early (z = 9) and later (z = 6) reionizations. We conclude that at z similar to 5.5, resolutions of 10kpc are necessary for convergence of LAF power at k < 0.1skm(-1), while higher-k modes require higher resolution, and that the timing of reionization does not significantly impact convergence given realistic photoheating.