On the Threshold of the Reionization Epoch

Discovery of the cosmic reionization epoch would represent a significantmilestone in cosmology. We present Keck spectroscopy of the quasar SDSS1044-0125, at z = 5.73. The spectrum shows a dramatic increase in the opticaldepth at observed wavelengths lambda >~7550 A, corresponding to z_abs >~ 5.2.Only a few small, narrow transmission regions are present in the spectrumbeyond that point, and out to the redshifts where the quasar signal begins. Weinterpret this result as a signature of the trailing edge of the cosmicreionization epoch, which we estimate to occur around ~ 6 (as indeedconfirmed by subsequent observations by Becker et al.), and extending down to z\~ 5.2. This behavior is expected in the modern theoretical models of thereionization era, which predict a patchy and gradual onset of reionization. Theremaining transmission windows we see may correspond to the individualreionization bubbles (Stromgren spheres) embedded in a still largely neutralintergalactic medium, intersected by the line of sight to the quasar. Futurespectroscopic observations of quasars at comparable or larger redshifts willprovide a more detailed insight into the structure and extent of thereionization era.Comment: Replaced with the revised version. To appear in The Astrophysical Journal Letters. Latex file, 13 pages, 3 eps figures included, AASTEX style files include