Ly-alpha absorbers at z<1 : HST-CFHT imaging and spectroscopy in the field of 3C 286

Abstract

We present further observational results on our Lya-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 3C286 (z_e = 0.849) with HST/FOS spectrum of the quasar between 1600 and 3000 A and HST/WFPC2 high spatial resolution imaging of some galaxies. Our results confirm our first conclusions on the nature of the intermediate redshift Ly-alpha forest absorbers. A small fraction of them is tightly linked to galaxies, and most probably arise in the external parts of giant halos (R~200 kpc with H0=50 km/s/Mpc) around fairly luminous galaxies (M_AB(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-alpha absorption line.