Chasing the Tail of Cosmic Reionization with Dark Gap Statistics in the
  Ly$\alpha$ Forest over $5 < z < 6$
  We present a new investigation of the intergalactic medium (IGM) near the end
of reionization using "dark gaps" in the Lyman-alpha (Ly$\alpha$) forest. Using
spectra of 55 QSOs at $z_{\rm em}>5.5$, including new data from the XQR-30 VLT
Large Programme, we identify gaps in the Ly$\alpha$ forest where the
transmission averaged over 1 comoving $h^{-1}\,{\rm Mpc}$ bins falls below 5%.
Nine ultra-long ($L > 80~h^{-1}\,{\rm Mpc}$) dark gaps are identified at $z<6$.
In addition, we quantify the fraction of QSO spectra exhibiting gaps longer
than $30~h^{-1}\,{\rm Mpc}$, $F_{30}$, as a function of redshift. We measure
$F_{30} \simeq 0.9$, 0.6, and 0.15 at $z = 6.0$, 5.8, and 5.6, respectively,
with the last of these long dark gaps persisting down to $z \simeq 5.3$.
Comparing our results with predictions from hydrodynamical simulations, we find
that the data are consistent with models wherein reionization extends
significantly below redshift six. Models wherein the IGM is essentially fully
reionized that retain large-scale fluctuations in the ionizing UV background at
$z \lesssim 6$ are also potentially consistent with the data. Overall, our
results suggest that signature of reionization in the form of islands of
neutral hydrogen and/or large-scale fluctuations in the ionizing background
remain present in the IGM until at least $z \simeq 5.3$.
