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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 |$\sigma _\zeta = 3.42$|⁠. [ABSTRACT FROM AUTHOR]

Neutral hydrogen (H  i) serves as a competitive tracer of the large scale structures, especially with the advent of more intensity mapping H  i surveys. In this work, we present a simulation-based framework to forecast the H  i power spectrum on non-linear scales (⁠|$k\gtrsim 1\ {\rm Mpc^{-1}}$|⁠), as measured by interferometer arrays like MeerKAT in the low-redshift (⁠|$z\le 1.0$|⁠) Universe. Building on a galaxy-based H  i mock catalogue, we meticulously consider various factors, including the emission line profiles of H  i discs and some observational settings, and explore their impacts on the H  i power spectrum. We find that the H  i power spectrum is relatively insensitive to the profile shape of H  i emission line at these scales, while showing a strong correlation with the profile width. We propose an empirical model to simulate the emission line profile width for each H  i source. The resulting H  i power spectrum is consistent with the results from the IllustrisTNG hydrodynamical simulation and follows the trend of the measurements obtained by MeerKAT at |$z\approx 0.44$|⁠ , though with a significantly lower amplitude. We demonstrate how the H  i abundance |$\Omega _{\rm HI}$| and the amplitude parameter in our width model can be constrained with the MeerKAT measurements, though a strong degeneracy is uncovered. Our work shows the potential to constrain statistical properties of H  i emission line profiles with future H  i intensity mapping experiments. [ABSTRACT FROM AUTHOR]

Cosmic reionization was driven by the imbalance between early sources and sinks of ionizing radiation, both of which were dominated by small-scale structure and are thus usually treated in cosmological reionization simulations by subgrid modelling. The recombination rate of intergalactic hydrogen is customarily boosted by a subgrid clumping factor , 〈 n 2〉/〈 n 〉2, which corrects for unresolved fluctuations in gas density n on scales below the grid-spacing of coarse-grained simulations. We investigate in detail the impact of this inhomogeneous subgrid clumping on reionization and its observables, as follows: (1) Previous attempts generally underestimated the clumping factor because of insufficient mass resolution. We perform a high-resolution N -body simulation that resolves haloes down to the pre-reionization Jeans mass to derive the time-dependent, spatially varying local clumping factor and a fitting formula for its correlation with local overdensity. (2) We then perform a large-scale N -body and radiative transfer simulation that accounts for this inhomogeneous subgrid clumping by applying this clumping factor-overdensity correlation. Boosting recombination significantly slows the expansion of ionized regions, which delays completion of reionization and suppresses 21 cm power spectra on large scales in the later stages of reionization. (3) We also consider a simplified prescription in which the globally averaged, time-evolving clumping factor from the same high-resolution N -body simulation is applied uniformly to all cells in the reionization simulation, instead. Observables computed with this model agree fairly well with those from the inhomogeneous clumping model, e.g. predicting 21 cm power spectra to within 20 per cent error, suggesting it may be a useful approximation. [ABSTRACT FROM AUTHOR]