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 fines in the spectrum of the quasar Q2126-158 (z(em) = 3.27) is presented. Regions free of absorption lines, in the Lyalpha forest of the quasar, are selected on the basis of high-resolution data (FWHM greater than or similar to 0.2 angstrom). We have extended the wavelength interval from 7000 to 8500 angstrom, by means of data taken at low resolution, to obtain a more reliable evaluation of the continuum shape redward of the Lyalpha emission, and we describe the procedure adopted to define the continuum level. We obtain a best estimate of tau = 0.013 +/- 0.026 at [z] = 3. Constraints on the baryon density of the intergalactic medium are discussed.