The impact of the IGM thermal state on the Lyα flux 3D power spectrum from linear to highly non-linear scales
Published in arXiv, 2026
Recently initiated and upcoming spectroscopic surveys, such as DESI and WST, will provide more than \(10^6\) high-\(z\) quasar spectra, enabling dense sky coverage by the Ly\(\alpha\) forest. This is expected to establish the three-dimensional (3D) Ly\(\alpha\) forest power spectrum, \(P_{\rm 3D,\alpha}\), as a probe of the thermal and ionization history of the intergalactic medium (IGM). To exploit this opportunity, we quantify the imprints of reionization on the post-reionization IGM using high-fidelity numerical models. We use the Sherwood and Sherwood–Relics cosmological hydrodynamical simulations with box sizes up to \(160\,h^{-1}\,\rm cMpc\) to investigate the impact of box size, mass resolution, and extracted grid resolution on \(P_{\rm 3D,\alpha}\) over \(2.4\leq z\leq4.8\). After applying a Zel’dovich control variate correction, simulation volume has a modest impact over most scales and orientations, whereas degrading the mass resolution produces differences of up to \(\sim13\%\). Insufficient resolution of the grid used for the optical-depth calculation can artificially enhance small-scale power by up to \(\sim35\%\). The timing of \HI reionization leaves only a percent-level imprint on \(P_{\rm 3D,\alpha}\) at \(z=2.4\), whereas varying the photoheating rate by a factor of two changes the large-scale power by \(\sim4\)–\(8\%\). Our results demonstrate that numerical effects can be comparable to, or exceed, the relic astrophysical signatures encoded in \(P_{\rm 3D,\alpha}\), making numerical convergence essential for interpreting precise Ly\(\alpha\) forest measurements. The strongest astrophysical imprint arises from spatially inhomogeneous \HI reionization, which enhances the large-scale power by up to \(\sim70\%\) at \(z=4.2\). This highlights the potential of post-reionization Ly\(\alpha\) forest measurements as a probe of the thermal history and spatial morphology of cosmic reionization.