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