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, the authors have found tentative evidence for H₂ lines at the same redshift (z = 2.811) as that of the well-known absorption-line system. They 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)×10¹⁶cm^-2; the fraction of H₂ appears to be about 2×10^-5. The ortho- to para-H₂ ratio corresponds to the excitation temperature of T_ex(H₂) = 300±150K, 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.