A HST spectroscopic study of QSOs with intermediate redshift damped Lyα systems

We present HST spectra for a sample of six QSOs with intermediate redshift (x(a) less than or equal to 1.) damped Ly alpha systems. These observations aim at measuring the H I column density and detect metal lines in order to investigate the metal enrichment of the gas, as well as the presence of neutral species, molecules and dust. All systems selected on the basis of 21 cm absorption and/or strong Fe II lines relative to Mg II ones turn out to have N(H I) larger than 10(20) cm(-2). From our detection of weak lines from minor metals and already published optical data, we determine relative abundances of Si, Mn, Fe, Ni, Zn. In PKS 1229-021, we measure [Zn/H] = -0.5 at z(a) = 0.3950 while in two other cases with intervening spiral galaxies and for which only [Fe/H] and [Mn/H] could be estimated, the metallicity could be close to solar. Thus, it appears that although the scatter of metallicities is as large at z(a) less than or equal to 1. as at high redshift, an increasing proportion of systems with metallicities similar or equal to 30% solar are found when going to lower redshifts. C I lines are tentatively detected in two systems. Given the low metallicity, the observed C I/H I ratio suggests that physical conditions in the absorbers are comparable to those in our Galaxy. In PKS 1229-021, the 21 cm absorption data combined with the new Ly alpha observations, imply a low temperature, T(s) less than or equal to 200 K, for the z(a) = 0.3950 absorbing gas. For the three systems in which they could be searched for, Ha molecules are not detected with an upper limit of about 10(18) cm(-2) on N(H(2)). No evidence is found for Galactic-type dust, except possibly in the 3C 286 z(a) = 0.6922 system. Our results suggest that available observations may be biased against dust-rich absorbers. Further, when all available measurements of N(H I) and [Zn/H] are considered, a clear deficiency of systems with large N(H I) and high metallicity is apparent. We conclude that dust extinction causes a preferential selection of QSOs with intervening gas relatively poor in metals, dust and molecules. As a consequence, the high end of the H I column density distribution (and hence Omega(g), the contribution of neutral gas to the cosmological mass density) is probably more heavily underestimated than previously thought, especially at low redshift. Such a bias could also explain the high incidence of non-spiral morphologies in our sample. We stress that observation of a larger sample of low z damped Ly alpha systems as well as surveys of damped Ly alpha systems in fainter QSOs would give a more representative view of the true diversity of absorber properties and should help to probe the denser phases of the interstellar medium in distant galaxies. Our program also provides a few new results on other (likely non-damped) metal systems. In PKS 0454+039, we detect for the first time Mg II absorption from a dwarf galaxy halo at z(a) = 0.072. Strong Mg II and Fe II absorption is also found in EX 0302-223 from a spiral galaxy at z = 0.118.