LYMAN-ALPHA ABSORPTION AND TIDAL DEBRIS

It is suggested that a significant fraction of the low column density absorption features seen in the spectra of quasars are produced in pre ssure-confined tidal debris that was built up in small groups and clus ters of galaxies over a Hubble time. We show that the space density an d cross section of tidal tails in groups of galaxies are large enough that they could constitute a major source of the low-redshift Lyalpha absorption features that are associated with galaxies. The space densi ty of groups within 10 Mpc of our galaxy is found to be 2.4 x 10(-3) M pc-3, which is close to the approximately 6 x 10(-3) Mpc-3 space densi ty calculated for Lyalpha absorbers, assuming they have a 1 Mpc radius . Other observational constraints on the properties of Lyalpha absorbe rs, such as their velocity dispersion, correlation properties, dimensi ons, and abundances, are shown to be consistent with this hypothesis.