Abstract

We have obtained spectra of 163 quasars at z(em) > 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 similar to 5 to model the average rest-frame Lyman continuum flux and to assess the mean free path lambda(912)(mfp) of the intergalactic medium to H I-ionizing radiation. At mean redshifts z(q) = (4.56, 4.86, 5.16), we measure lambda(912)(mfp) = (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 lambda(912)(mfp) = A[(1 + z)/5](eta) with A = (37 +/- 2)h(70)(-1) Mpc and eta = -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 lambda(912)(mfp) approximate to 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.