Probing Ultra-late Reionization: Direct Measurements of the Mean Free Path over $5<z<6$

Abstract

The mean free path of ionizing photons, $\hat{I}{»}_{\mathrm{mfp}}$, is a critical parameter for modeling the intergalactic medium (IGM) both during and after reionization. We present direct measurements of $\hat{I}{»}_{\mathrm{mfp}}$ from QSO spectra over the redshift range $5<z<6$, including the first measurements at $z\simeq 5.3$ and 5.6. Our sample includes data from the XQR-30 VLT large program, as well as new Keck/ESI observations of QSOs near $z\sim 5.5$, for which we also acquire new [C II] 158$\hat{I}¼$m redshifts with ALMA. By measuring the Lyman continuum transmission profile in stacked QSO spectra, we find $\hat{I}{»}_{\mathrm{mfp}}=9.33_{-1.80}^{+2.06}$, $5.40_{-1.40}^{+1.47}$, $3.31_{-1.34}^{+2.74}$, and $0.81_{-0.48}^{+0.73}$ pMpc at $z=5.08$, 5.31, 5.65, and 5.93, respectively. Our results demonstrate that $\hat{I}{»}_{\mathrm{mfp}}$ increases steadily and rapidly with time over $5<z<6$. Notably, we find that $\hat{I}{»}_{\mathrm{mfp}}$ 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 $\hat{I}{»}_{\mathrm{mfp}}$ constraints based on IGM Ly$\hat{I}\pm$ opacity, we find that the $\hat{I}{»}_{\mathrm{mfp}}$ evolution is consistent with scenarios wherein the IGM is still undergoing reionization and/or retains large fluctuations in the ionizing UV background well below redshift six.

17 pages, 6 figures, 2 tables; accepted for publication in ApJ