Molecular hydrogen, deuterium, and metal abundances in the damped Lyα system AT Zabs=3.025 toward Q0347-3819

We have detected in high-resolution spectra of the quasar Q0347-3819 obtained with the UV-Visual Echelle Spectrograph (UVES) at the VLT/Kueyen telescope over 80 absorption features in the Lyman and Werner H-2 bands at the redshift of a damped Lyalpha (DLA) system z(abs)=3.025. At the same redshift, numerous absorption lines of atoms and low ions (H I, D I, C II, C II*, N I, OI, Si II, P II, Ar I, Cr II, Fe II, and Zn II) were identified. The z(abs)=3.025 system spans over similar to80 km s(-1) and exhibits a multi-component velocity structure in the metal lines. The main component at z(abs)=3.02485 shows a total H-2 column density N(H-2)=(4.10+/-0.21) x 10(14) cm(-2) and a fractional molecular abundance f(H 2) = (1.94+/-0.10) x 10(-6) derived from the H-2 lines arising from the J=0-5 rotational levels of the ground electronic vibrational state. In the second strong component at z(abs)=3.024684 we were able to set a stringent upper limit to f(H 2) of 3.3x10(-9) (3 sigma c.l.). The population of the J levels can be represented by a single excitation temperature of T-ex = 825+/-1.10 K. The ortho-H-2: para-H-2 ratio equals 3.1+/-0.3. This ratio is comparable to that calculated from hot H-2 formation upon grain surfaces (ortho-H-2:para-H-2 = 3:1) but is higher than the "freezeout" quantity (ortho-H-2:para-H-2=1:4) predicted for cosmological epochs at z<20, implying recent formation of the hydrogen molecules following photodissociation of pristine H-2. In the main component the properties of the neutral atoms and low ions can be described with the same broadening parameter b similar or equal to 3 km s(-1) of the H-2 cloud. The rate at which UV photons are absorbed in the Lyman and Werner bands is found to be beta(0) similar or equal to 2x10(-9) s(-1). Taking into account that similar beta(0) values are observed in the Galactic diffuse molecular clouds, it shows that the UV radiation fields in the DLA and in the Milky Way are very much alike. We also found the H-2 formation rate of Rn-H similar or equal to 3x10(-1)6 s(-1) at z(abs)=3.025 which is consistent with observations in the Galactic disk diffuse clouds where n(H 2) is low. Comparing the relative rotational population ratios of H-2 with the measured C II*/C II ratio, we can infer that n(H) similar or equal to cm(-3) in the main component, which gives a size of D similar or equal to 14 pc along the line of sight. The mean value of the dust-to-gas ratio estimated for the whole z(abs)=3.025 system from the [Cr/Zn] ratio is equal to <(k)over tilde>=0.032+0.005 (in units of the mean Galactic interstellar medium value). For the first time we unambiguously reveal a pronounced [alpha-element/iron peak] enhancement of [O, Si/Zn] = 0.6+/-0.1 (1 sigma c.l.) at high redshift. The simultaneous analysis of metal and hydrogen lines leads to D/H = (3.75+/-0.25) x 10(-5). This value is consistent with standard big bang nucleosynthesis if the baryon-to-photon ratio, eta, lies within the range 4.37x10(-10) less than or similar to eta less than or similar to 5.32x10(-10), implying 0.016 less than or similar to Omega(b)h(100)(2) less than or similar to 0.020.