Reionization relics in the cross-correlation between the Lyα forest and 21 cm intensity mapping in the post-reionization era

ABSTRACT The tumultuous effects of ultraviolet photons that source cosmic reionization, the subsequent compression and shock-heating of low-density regions, and the modulation of baryons in shallow potential wells induced by the passage of ionization fronts, collectively introduce perturbations to the evolution of the intergalactic medium in the post-reionization era. These enduring fluctuations persist deep into the post-reionization era, casting a challenge upon precision cosmology endeavours targeting tracers in this cosmic era. Simultaneously, these relics from reionization also present a unique opportunity to glean insights into the astrophysics that govern the epoch of reionization. In this work, we propose a first study of the cross-correlation of Ly$\alpha$ forest and 21 cm intensity mapping, accounting for the repercussions of inhomogeneous reionization in the post-reionization era. We investigate the ability of Square Kilometre Array (SKA) $\times$ Dark Energy Spectroscopic Instrument (DESI)-like, SKA $\times$ MUltiplexed Survey Telescope (MUST)-like, and Packed Ultra-wideband Mapping Array (PUMA) $\times$ MUST-like instrumental set-ups to achieve a high-signal-to-noise ratio (SNR) in the redshift range $3.5 \le z \le 4$. Moreover, we assess how alterations in integration time, survey area, and reionization scenarios impact the SNR. Furthermore, we forecast the cross-correlation’s potential to constrain cosmological parameters under varying assumptions: considering or disregarding reionization relics, marginalizing over reionization astrophysics, and assuming perfect knowledge of reionization. Notably, our findings underscore the remarkable capability of a futuristic PUMA $\times$ MUST-like set-up, with a modest 100-h integration time over a 100 deg2 survey, to constrain the ionization efficiency error to $\sigma _\zeta = 3.42$.