The large-scale 21-cm power spectrum from reionization

TL;DR

This paper analyzes the 21-cm power spectrum during reionization, revealing a two-regime structure influenced by astrophysics and cosmology, and introduces a bias model linked to the mean free path of ionizing photons.

Contribution

It provides a detailed decomposition of the 21-cm power spectrum, linking large-scale bias to the mean free path and highlighting the impact of numerical implementation methods.

Findings

The 21-cm power spectrum transitions from matter-like to astrophysics-dominated at ~10% ionization.

The transition scale is approximately twice the mean free path of ionizing photons.

Implementation details of the mean free path significantly affect the power spectrum shape.

Abstract

Radio interferometers, such as the Low-Frequency Array and the future Square Kilometre Array, are attempting to measure the spherically averaged 21-cm power spectrum from the Epoch of Reionization. Understanding of the dominant physical processes which influence the power spectrum at each length-scale is therefore crucial for interpreting any future detection. We study a decomposition of the 21-cm power spectrum and quantify the evolution of its constituent terms for a set of numerical and semi-numerical simulations of a volume of $(714~\mathrm{Mpc})^3$, focusing on large scales with $k\lesssim 0.3$~Mpc$^{-1}$. We find that after $\sim 10$ per cent of the Universe has been ionized, the 21-cm power spectrum follows the power spectrum of neutral hydrogen fluctuations, which itself beyond a certain scale follows the matter power spectrum. Hence the signal has a two-regime form where the large-scale signal is a biased version of the cosmological density field, and the small-scale power spectrum is determined by the astrophysics of reionization. We construct a bias parameter to investigate the relation between the large-scale 21-cm signal and the cosmological density field. We find that the transition scale between the scale-independent and scale-dependent bias regimes is directly related to the value of the mean free path of ionizing photons ($\lambda_{\mathrm{MFP}}$), and is characterised by the empirical formula $k_{\mathrm{trans}} \approx 2/\lambda_{\mathrm{MFP}}$. Furthermore, we show that the numerical implementation of the mean free path effect has a significant impact on the shape of this transition. Most notably, the transition is more gradual if the mean free path effect is implemented as an absorption process rather than as a barrier.