We have obtained spectra of 163 quasars at z _ { em } > 4.4 with the Gemini Multi Object Spectrometers on the Gemini North and South telescopes , the largest publicly available sample of high-quality , low-resolution spectra at these redshifts .
From this homogeneous data set , we generated stacked quasar spectra in three redshift intervals at z \sim 5 .
We have modelled the flux below the rest-frame Lyman limit ( \lambda _ { \mathrm { r } } < 912 Å ) to assess the mean free path \lambda _ { mfp } ^ { 912 } of the intergalactic medium to H i -ionizing radiation .
At mean redshifts z _ { \mathrm { q } } = 4.56 , 4.86 and 5.16 , we measure \lambda _ { mfp } ^ { 912 } = \left ( 22.2 \pm 2.3 , 15.1 \pm 1.8 , 10.3 \pm 1.6 \right ) h _ { 70 } % ^ { -1 } proper Mpc with uncertainties dominated by sample variance .
Combining our results with \lambda _ { mfp } ^ { 912 } measurements from lower redshifts , the data are well modelled by a simple power-law \lambda _ { mfp } ^ { 912 } = A \left [ \left ( 1 + z \right ) / 5 \right ] ^ { \eta } with A = \left ( 37 \pm 2 \right ) h _ { 70 } ^ { -1 } Mpc and \eta = -5.4 \pm 0.4 between z = 2.3 and z = 5.5 .
This rapid evolution requires a physical mechanism – beyond cosmological expansion – which reduces the cosmic 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 consequently neutral fraction ) decreases with cosmic time .
Our measurements of the strongly redshift-dependent mean free path shortly after the completion of H i reionization serve as a valuable boundary condition for numerical models thereof .
Having measured \lambda _ { mfp } ^ { 912 } \approx 10 Mpc at z = 5.2 , we confirm that the intergalactic medium is highly ionized by that epoch and that the redshift evolution of the mean free path does not show a break that would indicate a recent end to H i reionization .