Tracing cosmic structure with neutral hydrogen after the Epoch of Reionization

TL;DR

This paper models the evolution of neutral hydrogen from the end of reionization to late-time structure formation using simulations of the 21-cm line, predicting observable power spectra for SKA-Low to constrain reionization and cosmology.

Contribution

It introduces a combined simulation approach to predict the 21-cm power spectrum during the transition period, accounting for late-time HI and inhomogeneous recombination effects.

Findings

Power spectrum drops by four orders of magnitude between redshifts 4 and 7.

Detectability of HI power spectrum at scales up to 1 h Mpc$^{-1}$ for 3 < z < 7 with SKA-Low.

Sufficient SNR for HI power spectrum to trace underlying halos for z < 5.

Abstract

We present a study of the transition of Neutral Hydrogen (HI) gas from the end of the Epoch of Reionization (EoR) to late-time large-scale structure. We examine the signature of the transition as traced through the redshifted 21-cm line with SKA-Low at $3 < z < 7$. To do so, we use the semi-numerical simulation \textsc{21cmFAST} to model the HI during the EoR and add a HI-halo based post-processing model of the late-time HI. This approach gives a robust estimate of the amplitude of the HI temperature field and predicts the observable power spectrum during the transition period. We find that our simulation pipeline reproduces the expected power spectrum trends from existing observations and theory, in addition to replicating current observational constraints on $\Omega_{\text{HI}}$. Our simulations predict a drop in power of four orders of magnitude between $4 < z < 7$. Assuming an inhomogeneous recombination model, we find a flattening of the power due to lingering neutral islands masking the late-time HI signal for $5 < z< 6.5$. Using SKA-Low deep survey parameters, we find HI power spectrum detectability at scales $k \leq 1$ $h$ Mpc$^{-1}$ for redshifts $3< z < 7$, even when using the horizon limit to mitigate foregrounds. Our results suggest a sufficient SNR of the HI power spectrum tracing the underlying halos $z < 5$, which can be used for late-time cosmology. Our results suggest that the resulting $\Omega_{\rm HI}$ constraints can trace different reionization scenarios such as a decreased escape fraction. This study implies that deep SKA-Low observations for $3< z< 7$ will be an important probe to constrain reionization parameters as well as cosmological models.