E. Giallongo et al., THE GUNN-PETERSON EFFECT IN THE SPECTRUM OF THE Z=4.7 QSO-1202-0725 -THE INTERGALACTIC MEDIUM AT VERY HIGH REDSHIFTS, The Astrophysical journal, 425(1), 1994, pp. 120000001-120000004
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 prese
nted. The relatively high resolution of the spectrum (approximately 40
km s-1) allows the selection of regions free of strong absorption lin
es 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 o
ptical depth at the highest absorption redshift observed at this resol
ution, z congruent-to 4.3. The derived baryon density of the intergala
ctic 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.