Lyman Absorption and Tidal Debris

It is suggested that a signi cant fraction of the low column-density absorption features seen in the spectra of quasars are produced in pressure-con ned tidal debris, that was built up in small groups and clusters of galaxies over a Hubble time. We show that the space-density and cross-section of tidal tails in groups of galaxies are large enough that they could constitute a major source of the low redshift Ly absorption features that are associated with galaxies. The space-density of groups within 10 Mpc of our galaxy is found to be 2.4 10 3 Mpc , which is close to the 6 10 3 Mpc 3 space-density calculated for Ly absorbers, assuming they have a 1 Mpc radius. Other observational constraints on the properties of Ly absorber such as their velocity dispersion, correlation properties, dimensions and abundances, are shown to be consistent with this hypothesis. Subject headings: Galaxies:Interactions | Galaxies:Quasars:Absorption Lines