The Lyα 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 1.25-10 h−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 Lyα power, convergence within 10% is achieved for k<0.1 s/km, 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∼5.5, resolutions of 10 kpc are necessary for convergence of LAF power at k<0.1 s/km, while higher-k modes require higher resolution, and that the timing of reionization does not significantly impact convergence given realistic photoheating.
@article{arxiv.2305.16200,
title = {Convergence of small scale Ly$\alpha$ structure at high-$z$ under different reionization scenarios},
author = {Caitlin C. Doughty and Joseph F. Hennawi and Frederick B. Davies and Zarija Lukić and Jose Oñorbe},
journal= {arXiv preprint arXiv:2305.16200},
year = {2023}
}