Limits on the Gunn-Peterson Effect at z = 5

Abstract

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 LRIS and HIRES spectrographs on the Keck telescopes. Lower resolution spectrophotometrically calibrated observations made with LRIS over the wavelength region 4500-9600 angstroms 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 D_A 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 tau = 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 ($J_ν \gg 4 \times 10^{-23}~{\rm ergs cm^{-2} s^{-1} Hz^{-1} sr^{-1}}$ at $z\sim4.72$) to produce a solution which is consistent with even minimum nucleosynthesis estimates of the baryon density. We also report the presence of a Mg II absorption system of extremely high equivalent width ($W_{λ,rest}(2796)=1.73$ Å) at z=2.304.