The impact of the IGM thermal state on the Ly$\alpha$ flux 3D power spectrum from linear to highly non-linear scales

Abstract

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, $P_{3D,\alpha }$, 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 $160h^{-1}\mathrm{cMpc}$ to investigate the impact of box size, mass resolution, and extracted grid resolution on $P_{3D,\alpha }$ over $2.4\le z\le 4.8$. 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 $\sim 13\%$. Insufficient resolution of the grid used for the optical-depth calculation can artificially enhance small-scale power by up to $\sim 35\%$. The timing of $\mathrm{H}\mathrm{I}$ reionization leaves only a percent-level imprint on $P_{3D,\alpha }$ at $z=2.4$, whereas varying the photoheating rate by a factor of two changes the large-scale power by $\sim 4-8\%$. Our results demonstrate that numerical effects can be comparable to, or exceed, the relic astrophysical signatures encoded in $P_{3D,\alpha }$, making numerical convergence essential for interpreting precise Ly$\alpha$ forest measurements. The strongest astrophysical imprint arises from spatially inhomogeneous $\mathrm{H}\mathrm{I}$ reionization, which enhances the large-scale power by up to $\sim 70\%$ 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.

32+21 pages, 14+7 figures, 1+6 tables, Comments welcome