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

The mean free path of ionizing photons, λ _mfp , is a critical parameter for modeling the intergalactic medium (IGM) both during and after reionization. We present direct measurements of λ _mfp from QSO spectra over the redshift range 5 < z < 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 μ m redshifts with ALMA. By measuring the Lyman continuum transmission profile in stacked QSO spectra, we find ${\lambda }_{\mathrm{mfp}}={9.33}_{-1.80}^{+2.06}$ , ${5.40}_{-1.40}^{+1.47}$ , ${3.31}_{-1.34}^{+2.74}$ , and ${0.81}_{-0.48}^{+0.73}$ pMpc at z = 5.08, 5.31, 5.65, and 5.93, respectively. Our results demonstrate that λ _mfp increases steadily and rapidly with time over 5 < z < 6. Notably, we find that λ _mfp 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 λ _mfp constraints based on IGM Ly α opacity, we find that the evolution of λ _mfp 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.