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A measure of the average depression between Lyman absorption lines in the spectrum of the faint quasar BR 1202-0725 (Zₑm_ = 4. 695) is presented. The relatively high resolution of the spectrum (~40 km s^-1^) allows the selection of regions free of strong absorption lines in the Lyα forest. A reliable evaluation of the continuum shape is based on the careful flux calibration and on the large wavelength interval covered (5000-9300 A). HERE A best estimate of τGP_ <= 0. 02 +/- 0. 03 has been found for the Gunn-Peterson optical depth at the highest absorption redshift observed at this resolution, z~4. 3. The derived baryon density of the intergalactic medium is OMEGAIGM_ <~ 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 ~ 5.
Giallongo et al. (Fri,) studied this question.