LYALPHA ABSORBERS AT Z 1 : HST-CFHT IMAGING AND SPECTROSCOPY IN THE FIELD OF 3C 286

We present further observational results on our Ly- only absorber identification project at low and intermediate redshifts. We combine CFHT/MOS imaging and spectroscopic observations of 25 galaxies in the field of the quasar 3C 286 (ze =0 :849) with HST/FOS spectrum of the quasar between 1600 and 3000 ˚ A and HST/WFPC2 high spatial resolution imag- ing of some galaxies. Our results confirm our first conclusions on the nature of the intermediate redshift Ly forest absorbers. A small fraction of them is tightly linked to galaxies, and most probably arise in the external parts of giant halos (R ' 200h 1 50 kpc) around fairly luminous galaxies (MAB(B) < 19) of various spectral and morphological types, as shown by HST imaging of these objects. Most of the absorbing clouds appear to be only "associated" with galaxies, in the sense that they could trace the gaseous filaments or sheets of large-scale structures. Thus, as expected from a sample tracing large scale structures, these galaxies have various spectral types. We also find that groups of galaxies have always at least one associated Ly absorption line.