A HST SPECTROSCOPIC STUDY OF QSOS WITH INTERMEDIATE REDSHIFT DAMPED LY-ALPHA SYSTEMS

We present HST spectra for a sample of six QSOs with intermediate reds hift (x(a) less than or equal to 1.) damped Ly alpha systems. These ob servations aim at measuring the H I column density and detect metal li nes in order to investigate the metal enrichment of the gas, as well a s the presence of neutral species, molecules and dust. All systems sel ected on the basis of 21 cm absorption and/or strong Fe II lines relat ive to Mg II ones turn out to have N(H I) larger than 10(20) cm(-2). F rom our detection of weak lines from minor metals and already publishe d 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 [F e/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 incr easing proportion of systems with metallicities similar or equal to 30 % solar are found when going to lower redshifts. C I lines are tentati vely detected in two systems. Given the low metallicity, the observed C I/H I ratio suggests that physical conditions in the absorbers are c omparable to those in our Galaxy. In PKS 1229-021, the 21 cm absorptio n data combined with the new Ly alpha observations, imply a low temper ature, T-s less than or equal to 200 K, for the z(a) = 0.3950 absorbin g gas. For the three systems in which they could be searched for, Ha m olecules are not detected with an upper limit of about 10(18) cm(-2) o n 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 availabl e observations may be biased against dust-rich absorbers. Further, whe n all available measurements of N(H I) and [Zn/H] are considered, a cl ear deficiency of systems with large N(H I) and high metallicity is ap parent. We conclude that dust extinction causes a preferential selecti on of QSOs with intervening gas relatively poor in metals, dust and mo lecules. As a consequence, the high end of the H I column density dist ribution (and hence Omega(g), the contribution of neutral gas to the c osmological mass density) is probably more heavily underestimated than previously thought, especially at low redshift. Such a bias could als o explain the high incidence of non-spiral morphologies in our sample. We stress that observation of a larger sample of low z damped Ly alph a systems as well as surveys of damped Ly alpha systems in fainter QSO s would give a more representative view of the true diversity of absor ber properties and should help to probe the denser phases of the inter stellar medium in distant galaxies. Our program also provides a few ne w 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 hal o 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.