UVES observations of QSO 0000 – 2620 : molecular hydrogen abundance in the damped Ly α system at z abs = 3 . 3901 ?

We have discovered molecular hydrogen in a fourth quasar damped Lyα system (hereafter DLA). The UVES spec-trograph on the 8.2m ESO Kueyen telescope has allowed the detection of H 2 in gas with low metallicity, Z/Z 10 −2 , and high neutral hydrogen column density, N (H i) 2.6 × 10 21 cm −2 , at redshift z abs = 3.3901 toward QSO 0000–2620. The measured H 2 fractional abundance of f (H 2) 4 × 10 −8 is lower than a typical value for Galactic interstellar clouds of high N (H i) column density by a factor of (2 − 3) × 10 6. Since H 2 molecules are formed efficiently on dust grains, it implies that the dust condensation in this DLA is negligible, and hence the abundances derived from metal absorption lines are the actual ones. The obtained f (H 2) value leads to an estimate of the dust number density of n d DLA ∼ 10 −3 n d ISM , which is consistent with the dust-togas ratiõ k ≤ 1.6 × 10 −3 derived independently from the [Cr/Zn] and [Fe/Zn] ratios.