Convergence of the simulated Lyman alpha forest at z>5
The Lyman alpha forest at very high z (>5), arising from gas in the intergalactic medium (IGM), can serve as a probe of the thermal history of the IGM, the metagalactic ionizing background, dark matter characteristics, and reionization. Depending on the degree of pre-reionization X-ray heating, the IGM could potentially cool to temperatures on the order of <10 K by z=10, allowing for the formation of sub-kpc structure, far smaller than the spatial resolutions of typical cosmological simulations used in investigations of IGM structure. With evidence mounting for an extremely late end to reionization (z<6), which limits the time available for thermodynamic relaxation after photoheating, the pre-reionization small scale structure may survive longer than anticipated, tightening the resolution requirements to accurately model the post-reionization IGM. To characterize the impact of reionization’s timing on convergence, we perform a set of simulations using the Nyx code, exploring the interaction with box size and spatial resolution on the properties of the matter distribution and the simulated Lyman alpha forest. We find that the measured pressure smoothing scale and Lyman alpha power at small physical scales (log k/(s/km) > -1.0) still differ by ~20 percent when increasing the grid resolution from 10 kpc to 5 kpc. Further, while the mean flux is converged for <10 kpc for a given reionization history, the value can differ by tens of percent between an early (z=9) and a late (z=5.5) reionization timing. We conclude that the 10-20 kpc resolution commonly used in cosmological simulations of the IGM may be inadequate for predicting the impact of reionization on small scale Lyman alpha power, especially in the case of late reionization.
