Limits on the Gunn-Peterson Effect at z = 5

We report new limits on the Gunn-Peterson effect at a redshift near 5 using spectroscopic observations of the z = 5 Sloan Digital Sky Survey quasar, J033829.31+002156.3, made with the Low Resolution Imaging Spectrometer (LRIS) and the High-Resolution Echelle Spectrometer (HIRES) on the Keck telescopes. Lower resolution spectrophotometrically calibrated observations made with LRIS over the wavelength region 4500-9600 Å were used to obtain a continuum shape and to flux-calibrate much higher resolution (R = 36,000) observations made with HIRES. The LRIS data show an Oke DA index of 0.75. Portions of the HIRES spectrum return to near the extrapolated continuum level. Including both statistical and systematic errors, we place an upper limit of τ = 0.1 on the regions of minimum opacity. We argue that, even if this opacity arises in underdense regions of the intergalactic gas, we require a high value of the metagalactic ionizing flux at these redshifts (Jν ≫ 4 × 10-23 ergs cm-2 s-1 Hz-1 sr-1 at z ~ 4.72) to produce a solution which is consistent with even minimum nucleosynthesis estimates of the baryon density. We also report the presence of an Mg II absorption system of extremely high equivalent width [Wλ, rest(2796) = 1.73 Å] at z = 2.304.