A direct measure of the Gunn-Peterson effect at Z = 3 - Limits on the baryon density of the intergalactic medium at high redshifts

A measure of the average depression between Lyman absorption lines in the spectrum of the quasar Q2126 - 158 (Z_em_ = 3.27) is presented. Regions free of absorption lines, in the Lyα forest of the quasar, are selected on the basis of high-resolution data (FWHM > 0.2 A). We have extended the wavelength interval from 7000 to 8500 A, by means of data taken at low resolution, to obtain a more reliable evaluation of the continuum shape redward of the Lyα emission, and we describe the procedure adopted to define the continuum level. We obtain a best estimate of τ = 0.013 +/- 0.026 at = 3. Constraints on the baryon density of the intergalactic medium are discussed.