Molecular hydrogen in the z = 2.811 absorbing material toward the quasar PKS 0528-250

Among the previously unidentified absorption features in the spectrum of the quasar PKS 0528–250 obtained by Morton et al. 1980, we have found tentative evidence for H2 lines at the same redshift (z = 2.811) as that of the well-known absorption-line system. We have estimated the column density of molecules from the curve of growth derived for this system by Morton et al. N(H2) = (2.9 ± 0.3) × 1016 cm−2; the fraction of H2 appears to be about 2 × 10−5. The ortho- to para-H2 ratio corresponds to the excitation temperature of Tex (H2) = 300 ± 150 K, though the upper rotational levels |J 3| turn out to be overpopulated. The results obtained seem to provide substantial evidence for the existence of molecular hydrogen at the early cosmological epoch with z ∼ 3.