The Giant Gemini GMOS survey of z>4.4 quasars - I. Measuring the mean free path across cosmic time

We have obtained spectra of 163 quasars at $z_{\mathrm{e}\mathrm{m}}>4.4$ 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 (${\lambda }_{\mathrm{r}}<912$\AA) to assess the mean free path ${\lambda }_{\mathrm{m}\mathrm{f}\mathrm{p}}^{912}$ of the intergalactic medium to HI-ionizing radiation. At mean redshifts $z_{\mathrm{q}}=4.56$, 4.86 and 5.16, we measure $\lambda_$\mathrm{m}\mathrm{f}\mathrm{p}$^{912}=(22.2\pm 2.3, 15.1\pm 1.8, 10.3\pm 1.6)h_{70}^{-1}$ proper Mpc with uncertainties dominated by sample variance. Combining our results with ${\lambda }_{\mathrm{m}\mathrm{f}\mathrm{p}}^{912}$ measurements from lower redshifts, the data are well modelled by a simple power-law ${\lambda }_{\mathrm{m}\mathrm{f}\mathrm{p}}^{912}=A[(1+z)\mathrm{/}5{]}^{\eta }$ with $A=(37\pm 2)h_{70}^{-1}$ 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 HI 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 HI reionization serve as a valuable boundary condition for numerical models thereof. Having measured ${\lambda }_{\mathrm{m}\mathrm{f}\mathrm{p}}^{912}\approx 10$ 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 HI reionization.