On the Threshold of the Reionization Epoch

Discovery of the cosmic reionization epoch would represent a significant milestone in cosmology. We present Keck spectroscopy of the quasar SDSS 1044-0125, at z = 5. 73. The spectrum shows a dramatic increase in the optical depth at observed wavelengths >~ 7550 A, corresponding to zₐbs >~ 5. 2. Only a few small, narrow transmission regions are present in the spectrum beyond that point, and out to the redshifts where the quasar signal begins. We interpret this result as a signature of the trailing edge of the cosmic reionization epoch, which we estimate to occur around ~ 6, and extending down to z ~ 5. 2. This behavior is expected in at least some modern theoretical models of the reionization era, which predict a patchy and gradual onset of reionization. The remaining transmission windows would correspond to individual reionization bubbles (Stromgren spheres) embedded in a still largely neutral intergalactic medium, intersected by the line of sight to the quasar. This interpretation can be tested by future spectroscopic observations of quasars at comparable or larger redshifts.