Lyman Alpha Absorption and Tidal Debris
  The origin and evolution of structure in the Universe is one of the major questions occupying astronomers today. An understanding of the Lyalpha absorbers seen in QSO spectra is an important part of this program since such absorbers can be traced back to very high redshifts. Their mere existence places constraints on the physical state of the intergalactic medium. The discovery of Lyalpha absorbers at low redshift allows us to estimate for the first time what fraction of low redshift Lyalpha absorbers are (i) randomly distributed, (ii) distributed like galaxies but not physically associated with luminous objects, (iii) actually part of the halos of luminous galaxies, or (iv) tidal tails within galaxy groups. Results from the sightline to the QSO 3C273 suggest that the majority of the absorbers are not associated with galaxies, but that there is a significant subset that are. The absorbers associated with galaxies may be produced in enormous gaseous disks surrounding normal spiral galaxies, or may be tidal material bound up in small groups of galaxies 
Simon L. Morris