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Our understanding of the intergalactic medium at redshifts $z=5$-$6$ hasimproved considerably in the last few years due to the discovery of quasarswith $z>6$ that enable Lyman-$\alpha$ forest studies at these redshifts. Arealisation from this has been that hydrogen reionization could end much laterthan previously thought, so that large "islands" of cold, neutral hydrogencould exist in the IGM at redshifts $z=5$-$6$. By using radiative transfersimulations of the IGM, we consider the implications of the presence of theseneutral hydrogen islands for the 21-cm power spectrum signal and its potentialdetection by experiments such as HERA, SKA, LOFAR, and MWA. In contrast withprevious models of the 21-cm signal, we find that thanks to the late end ofreionization 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 valueof the power spectrum is several orders of magnitude higher than that in theconventional models considered in the literature for these redshifts. Such highvalues of the 21-cm power spectrum should be detectable by HERA and SKA1-LOW in$\sim 1000$ hours, assuming optimistic foreground subtraction. This redshiftrange is also attractive due to relatively low sky temperature and potentiallygreater abundance of multiwavelength data.

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