Chasing the Tail of Cosmic Reionization with Dark Gap Statistics in the Lyα Forest over 5 < z < 6

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