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

Abstract 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 from QSO spectra over redshift range 5 < z 6, including first at ≃ 5.3 5.6. Our sample includes data XQR-30 VLT large program, as well new Keck/ESI observations QSOs near ∼ 5.5, which we also acquire [C ii ] 158 μ m redshifts with ALMA. By measuring Lyman continuum transmission profile in stacked spectra, find λ mfp = 9.33 − 1.80 + 2.06 ${5.40}_{-1.40}^{+1.47}$?> 5.40 1.40 1.47 ${3.31}_{-1.34}^{+2.74}$?> 3.31 1.34 2.74 ${0.81}_{-0.48}^{+0.73}$?> 0.81 0.48 0.73 pMpc = 5.08, 5.31, 5.65, 5.93, respectively. results demonstrate that increases steadily rapidly time 6. Notably, deviates significantly predictions based on fully ionized relaxed IGM late 5.3. comparing our to model indirect constraints Ly α opacity, evolution consistent scenarios wherein still undergoing reionization and/or retains fluctuations UV background below