We have discovered a fourth quasar damped Ly-alpha system (hereafter DLA) which contains molecular hydrogen. The UVES spectrograph on the 8.2m ESO Kueyen telescope has allowed the detection of H₂ in gas with low metallicity, Z/Z_Sun ~ 10^-2 and high neutral hydrogen column density, N(H) ~ 2.6^.10²¹ cm^-2, at redshift z_abs = 3.3901 toward QSO 0000-2620. The measured H₂ fractional abundance of f(H₂) ~ 4^.10^-8 is lower than a typical Galactic value for high N(H) interstellar clouds by a factor of (2-3)^.10⁶. Since H₂ molecules are formed efficiently on dust grains, it implies that the dust condensation in this DLA is negligible, and, hence, the derived gas-phase metal abundances are actual. The obtained f(H₂) 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-to-gas ratio k < 1.6^.10^-3 derived independently from the [Cr/Zn] and [Fe/Zn] ratios. Low molecularization of the z_abs = 3.3901 absorbing material resembles the low molecular content of dwarf galaxies and thus favors the assumption that this DLA is also a dwarf galaxy.

Page design by Alexander Potekhin (palex.astro@mail.ioffe.ru).
Page created on June 19, 2000.