The giant gemini GMOS survey of zem > 4.4 quasars - I. Measuring the mean free path across cosmic time

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 λ912mfp of the intergalactic medium to H I-ionizing radiation. At mean redshifts zq =(4.56, 4.86, 5.16), we measure λ912mfp = (22.2 ± 2.3, 15.1 ± 1.8, 10.3 ± 1.6) h-1 70 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 λ912mfp = A[(1 + z) /5]η with A = (37 ± 2) h-1 70 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 λ912mfp ≈ 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.