Probing Ultralate Reionization: Direct Measurements of the Mean Free Path over 5 < z < 6

The mean free path of ionizing photons, λmfp , is a critical parameter for modeling the intergalactic medium (IGM) both during and after reionization. We present direct measurements of λmfp from QSO spectra over the redshift range 5 &lt; z &lt; 6, including the first measurements at z ; 5.3 and 5.6. Our sample includes data from the XQR-30 VLT large program, as well as new Keck/ESI observations of QSOs near z ∼ 5.5, for which we also acquire new [C II] 158 μm redshifts with ALMA. By measuring the Lyman continuum transmission profile in stacked QSO spectra, we find l = - + 9.33mfp 1.80 2.06, - + 5.40 1.40 1.47, - + 3.31 1.34 2.74, and - + 0.81 0.48 0.73 pMpc at z = 5.08, 5.31, 5.65, and 5.93, respectively. Our results demonstrate that λmfp increases steadily and rapidly with time over 5 &lt; z &lt; 6. Notably, we find that λmfp deviates significantly from predictions based on a fully ionized and relaxed IGM as late as z = 5.3. By comparing our results to model predictions and indirect λmfp constraints based on IGM Lyα opacity, we find that the evolution of λmfp is consistent with scenarios wherein the IGM is still undergoing reionization and/or retains large fluctuations in the ionizing UV background well below redshift 6.