2.1. Atomic Gas
The most striking feature in interacting galaxy groups is the presence of large tidal tails of matter dragged out of the galaxies, huge HI extensions with respect to the optical systems. Yun et al. (1993) have found large quantities of HI all around the M81 / M82 / NGC 3077 system, and Hibbard (1995) shows in his thesis an evolving sequence of interacting/merging galaxies, where the HI extensions are conspicuous. More precisely, the percentage of the total HI found in the tails/extensions is increasing with the merging stage, from 20% in the M81 system, to 80% in the merger remnant NGC 7252. This does not mean that most of the gas will be expelled from interacting systems. In fact, with all probability, the gas dragged out remains bound to the system, and will rain back onto the merger remnant, after some billion years. Hibbard et al. (1994) show that the gas at the bottom of the tails in NGC 7252 is infalling.
In normal galaxies, large HI extents are very rare (Briggs et al 1980, Broeils 1992, Hoffman et al 1996). In average the gas extent is RHI ~ 2 Ropt. Only interacting systems show extended gas.