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

TL;DR

This study investigates how simulation resolution and reionization timing affect the convergence of Ly$ ext{alpha}$ forest models at high redshift, emphasizing the importance of small-scale structures and thermal effects.

Contribution

It provides detailed convergence analysis of hydrodynamical simulations for Ly$ ext{alpha}$ forest at high redshift under different reionization scenarios, highlighting resolution requirements.

Findings

Convergence within 10% for k<0.1 s/km at 10 kpc resolution.

Larger k modes need higher resolution for accurate modeling.

Reionization timing has minimal impact on convergence at z~5.5.

Abstract

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 1.25-10 $h^{-1}$ 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 Ly$\alpha$ 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 $z\sim5.5$, 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.