Implications of the $z>5$ Lyman-$\alpha$ forest for the 21-cm power spectrum from the epoch of reionization

TL;DR

This paper explores how late reionization and neutral hydrogen islands at redshifts 5-6 could produce a stronger 21-cm signal than previously thought, making detection by current experiments more feasible.

Contribution

It introduces radiative transfer simulations showing that late reionization results in higher 21-cm power spectrum signals at z=5-6 than earlier models predicted.

Findings

21-cm power spectrum remains high (~10 mK^2) at z=5-6

High 21-cm power is detectable by HERA and SKA1-LOW within 1000 hours

Neutral hydrogen islands significantly impact the 21-cm signal during late reionization

Abstract

Our understanding of the intergalactic medium at redshifts $z=5$-$6$ has improved considerably in the last few years due to the discovery of quasars with $z>6$ that enable Lyman-$\alpha$ forest studies at these redshifts. A realisation from this has been that hydrogen reionization could end much later than previously thought, so that large "islands" of cold, neutral hydrogen could exist in the IGM at redshifts $z=5$-$6$. By using radiative transfer simulations of the IGM, we consider the implications of the presence of these neutral hydrogen islands for the 21-cm power spectrum signal and its potential detection by experiments such as HERA, SKA, LOFAR, and MWA. In contrast with previous models of the 21-cm signal, we find that thanks to the late end of reionization the 21-cm power in our simulation continues to be as high as $\Delta^2_{21}=10~\mathrm{mK}^2$ at $k\sim 0.1~h/$cMpc at $z=5$-$6$. This value of the power spectrum is several orders of magnitude higher than that in the conventional models considered in the literature for these redshifts. Such high values of the 21-cm power spectrum should be detectable by HERA and SKA1-LOW in $\sim 1000$ hours, assuming optimistic foreground subtraction. This redshift range is also attractive due to relatively low sky temperature and potentially greater abundance of multiwavelength data.