THE GUNN-PETERSON EFFECT IN THE SPECTRUM OF THE Z=4.7 QSO-1202-0725 - THE INTERGALACTIC MEDIUM AT VERY HIGH REDSHIFTS

A measure of the average depression between Lyman absorption lines in the spectrum of the faint quasar BR 1202-0725 (Z(em) = 4.695) is presented. The relatively high resolution of the spectrum (approximately 40 km s-1) allows the selection of regions free of strong absorption lines in the Lyalpha forest. A reliable evaluation of the continuum shape is based on the careful flux calibration and on the large wavelength interval covered (5000-9300 angstrom). A best estimate of tau(GP) less-than-or-equal-to 0.02 +/- 0.03 has been found for the Gunn-Peterson optical depth at the highest absorption redshift observed at this resolution, z congruent-to 4.3. The derived baryon density of the intergalactic medium is OMEGA(IGM) approximately-less-than 0.01 if the observed quasars are the major contributor to the ionizing UV background flux. This limit, when compared with the total baryon density deduced from the nucleosynthesis, could imply that most of the baryons are already in bound systems at z approximately 5.