We have discovered a new molecular hydrogen cloud in the damped Ly-alpha system at z = 3.025 toward QSO 0347-3819. The main component at z = 3.024855 shows N(H₂) = 3.4 10¹⁴ cm^-2 and a fractional molecular abundance f(H₂) = 1.6 10^-6. The excitation temperature of T_ex = 800 K is estimated. The ortho:para-H₂ ratio equals 2.7 +/- 0.5. This ratio is comparable to that calculated from hot H₂ formation upon grain surfaces (ortho:para-H₂ = 3:1), but it is higher than the `freeze-out' quantity (ortho:para-H₂ = 1:4) predicted for cosmological epochs at z < 20, implying recent formation of the hydrogen molecules following photo-dissociation of pristine H₂. The calculated photo-absorption rate of beta₀ = 6 10^-10 s^-1 implies that the UV radiation field is alike the Galactic one. The H₂ formation rate Rn_H = 4 10^-16 s^-1 is shown to be consistent with observations in the MW diffuse clouds with low n_H2. From the analysis of the fine-structure lines of CII and the population of the low rotational levels of H₂, we measured for the first time at z > 3 the temperature of the cosmic background radiation of T_CMBR = 11.7^+1.6_-2.5 K. This result supports T_CMBR = 11.0 K predicted by the hot Big Bang cosmology at z = 3.025. The simultaneous analysis of metal and hydrogen lines leads to the hydrigen isotopic ratio of D/H = 3.2 +/- 0.4 10^-5. It implies the present-day baryon density Omega_b h² = 0.017 - 0.022, which is in good agreement with a new analysis of the BOOMERANG experiment yielding Omega_b h² = 0.021 +/- 0.003.

Page created by Alexander Potekhin (palex.astro@mail.ioffe.ru) on June 19, 2001.