2014MNRAS.445.1745W

We have obtained spectra of 163 quasars at zem > 4.4 with the Gemini Multi Object Spectrometers, the largest publicly available sample of high-quality, low-resolution spectra at these redshifts. From this data set, we generated stacked quasar spectra in three redshift intervals at z ∼ 5 to model the average rest-frame Lyman continuum flux and to assess the mean free path λ_mfp^912 of the intergalactic medium to H i-ionizing radiation. At mean redshifts zq=(4.56, 4.86, 5.16), we measure λ_mfp^912=(22.2 ±2.3, 15.1 ±1.8, 10.3 ±1.6)h_70^-1 proper Mpc with uncertainties dominated by sample variance. Combining our results with measurements from lower redshifts, the data are well modelled by a power law λ_mfp^912=A[(1+z)/5]^η with A=(37 ±2)h_70^-1 Mpc and η=-5.4±0.4 at 2.3 < z < 5.5. This rapid evolution requires a physical mechanism - beyond cosmological expansion - which reduces the effective Lyman limit opacity. We speculate that the majority of H i Lyman limit opacity manifests in gas outside galactic dark matter haloes, tracing large-scale structures (e.g. filaments) whose average density and neutral fraction decreases with cosmic time. Our measurements of the mean free path shortly after H i reionization serve as a valuable boundary condition for numerical models thereof. Our measured λ_mfp^912~10 Mpc at z=5.2 confirms that the intergalactic medium is highly ionized without evidence for a break that would indicate a recent end to H i reionization.