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 pressure-confined 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$\alpha$ absorption features that are associated with galaxies. The space-density of groups within 10 Mpc of our galaxy is found to be 2.4 $\times 10^{-3}$ Mpc$^{-3}$, which is close to the $\sim 6 \times 10^{-3}$ Mpc$^{-3}$ space-density calculated for Ly$\alpha$ absorbers, assuming they have a 1 Mpc radius. Other observational constraints on the properties of Ly$\alpha$ absorber such as their velocity dispersion, correlation properties, dimensions and abundances, are shown to be consistent with this hypothesis.