The mean free path of ionizing photons at 5 < z < 6: evidence for rapid evolution near reionization

The mean free path of ionizing photons, λ_mfp, is a key factor in the photoionization of the intergalactic medium (IGM). At z ≳ 5, however, λ_mfp may be short enough that measurements towards QSOs are biased by the QSO proximity effect. We present new direct measurements of λ_mfp that address this bias and extend up to z ~ 6 for the first time. Our measurements at z ~ 5 are based on data from the Giant Gemini GMOS survey and new Keck LRIS observations of low-luminosity QSOs. At z ~ 6 we use QSO spectra from Keck ESI and VLT X-Shooter. We measure $\lambda _{\rm mfp} = 9.09^{+1.62}_{-1.28}$ proper Mpc and $0.75^{+0.65}_{-0.45}$ proper Mpc (68 per cent confidence) at z = 5.1 and 6.0, respectively. The results at z = 5.1 are consistent with existing measurements, suggesting that bias from the proximity effect is minor at this redshift. At z = 6.0, however, we find that neglecting the proximity effect biases the result high by a factor of two or more. Our measurement at z = 6.0 falls well below extrapolations from lower redshifts, indicating rapid evolution in λ_mfp over 5 < z < 6. This evolution disfavours models in which reionization ended early enough that the IGM had time to fully relax hydrodynamically by z = 6, but is qualitatively consistent with models wherein reionization completed at z = 6 or even significantly later. Our mean free path results are most consistent with late reionization models wherein the IGM is still 20 per cent neutral at z = 6, although our measurement at z = 6.0 is even lower than these models prefer.