In nearby loose groups of galaxies, HI appendages are often seen to extend outward from the disks of member galaxies: peninsular "plumes" and "tails," connecting "bridges" and other such features. A number (but not all) of these HI appendages have been explained in terms of tidal interactions among neighboring galaxies. In the Local Group, the linear feature known as the Magellanic Stream is probably the result of tidal forces in the Milky Way-LMC-SMC system. Examples of HI tidal streams which have been successfully modeled include those seen in the systems NGC 4631 / 56, NGC 4038 / 9, and M81 / M82 / NGC 3077. Figure 12.5 shows the HI distribution in the Leo triplet NGC 3623 / 7 / 8 obtained at Arecibo. In this example, the overall characteristics of the observed HI distribution can be well reproduced by the interaction caused by postulating a hyperbolic passage of NGC 3627 past its neighbor NGC 3628 with a perigalactic distance of about 20 kpc. The disruption caused to NGC 3627 is much reduced because the sense of its orbital motion is opposite to that of its rotation; in NGC 3628, on the other hand, the two vectors are aligned and the damage is more severe, resulting in the conspicuous tail drawn mainly from the periphery of that galaxy. NGC 3623 has not recently had a close interaction with the other two.
Figure 12.5. HI distribution in the Leo triplet of galaxies - NGC 3623, NGC 3627, and NGC 3628 - obtained with the Arecibo 305-m reflector by Haynes et al. (1979). Angular resolution of the map is 3.3'. The contours are of neutral hydrogen column density in units of 3.1 × 1018 cm-2.
Several systems which contain substantial amounts of HI outside of galactic disks are less easily modeled as transient phenomena. Stefan's quintet (Shostak et al. 1984) is a group of five objects located close together on the sky. One of the galaxies is likely to be in the foreground; the other four lie at the same redshift and are morphologically peculiar. In fact, the HI at the velocity of the suspected foreground galaxy is regular while at least three distinct velocity systems are seen at the higher redshift. The bulk of the HI at the higher velocities lies outside the optical boundaries of any of the remaining quartet, and it appears that tidal and collisional stripping among those galaxies have succeeded in removing the majority of the disk gas from the participants. An even more perplexing case is found in the ring of HI found in the multiple group dominated by M96 and M105, also in Leo (Schneider et al. 1986).