The nature of intermediate-redshift damped Lyα absorbers

We present HST/WFPC2 high-spatial resolution images in the R and B bands of the close environment of the sightlines to seven quasars which spectra show either a damped Lyα absorption line, 21 cm absorption, or a very strong Mg ii/Fe ii absorption system at intermediate redshifts (0.4 ≤ z ≤ 1). Objects down to about 0.3′′, or 2.0 kpc at z = 0.6 (H0 = 50 kms s−1 Mpc−1, q0 = 0), and to a limiting magnitude m702,lim = 25.9 could be detected for seven fields comprising eight absorbers (one at higher redshift z = 1.78 towards MC 1331+170) with high H i column densities of at least 1× 1020 cm−2. In each case, a candidate absorber with absolute magnitude MB ' −19.0 or much brighter has been detected. This small sample of gas-rich galaxies at intermediate redshifts covers a wide range in morphological types. There are three spiral galaxies of various sizes and luminosities (towards 3C 196, Q 1209+107 and MC 1331+170), three compact objects (towards EX 0302−223, PKS 0454+039 and, at high redshift, MC 1331+170), and two amorphous, low surface brightness galaxies (towards PKS 1229−021 and 3C 286). In the fields around 3C 196, PKS 1229−021 and Q 1209+107, there is an excess of galaxies in the PC2 images, suggestive of the presence of a group of galaxies associatedwith the damped Lyα absorber, or maybe with the quasar itself for the two ze ≤ 1.0 cases. For 3C 196 and 3C 286, the quasar host galaxies have also tentatively been discovered. We do not detect any quasar multiple images, implying no large amount of dark matter around the damped Lyα absorbers. This survey also led to the discovery of the first z ' 1.0 optical counterpart of a quasar radio jet (PKS 1229−021). As will be reported elsewhere (Boissé et al. 1996), spectroscopy with the HST-FOS of the strongMg ii/Fe ii absorption systems confirms the validity of our selection criterion in predicting the existence of damped Lyα systems. Send offprint requests to: V. Le Brun ? Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by theAssociation ofUniversities for Research inAstronomy, Inc., under NASA contract NAS 5-26555