Annu. Rev. Astron. Astrophys. 1978. 16: 103-39 Copyright © 1978 by Annual Reviews. All rights reserved

4.6. Kinematical Effects of Tidal Interactions

We close this travelogue of disk-galaxy kinematics from the nuclei to the peripheries with a brief summary of the results on tidally interacting systems. In contrast to the case of warps discussed above, there is here no doubt that more than one galaxy is involved. Toomre & Toomre (1972) have been very successful in representing the geometry of four examples with models of gravitationally induced tidal disturbances that result in the formation of bridges and tails. The kinematical predictions of these models have been largely confirmed by spectroscopic observations of Arp 295 (Stockton 1974b), NGC 4676 (Stockton 1974a), and NGC 5195 (Schweizer 1977). The existence of HI in the optical tails of NGC 4038/39 was first reported by Huchtmeier & Bohnenstengel (1975), and a detailed study of the HI kinematics has been recently completed by van der Hulst (1977, 1978). In this last example the observations are in very good agreement with the model when a reasonable adjustment of the parameters is made.

The M81 group has also been examined in detail. Roberts (1972) and Davies (1974) were the first to show that HI is distributed extensively between the triplet of galaxies M81, M82, and NGC 3077. Cottrell (1976a) found the HI in NGC 3077 to be strongly distorted and displaced about 4 kpc from the optical center; a computer simulation showed that this could be the result of a close encounter with M81 about 2 to 6 × 10⁸ years ago. In a more sensitive survey of the area between M81 and NGC 3077, van der Hulst (1977, 1978) discovered a bridge of HI containing narrow filaments ( 3 kpc wide) which seems to extend from the outermost western arm of M81 around to the south and then eastward to join NGC 3077, 45 kpc away. The shape and kinematics of this bridge could be approximately represented by a simplified computer model of tidal interactions between the members of the triplet a few times 10⁸ years ago. Van der Hulst also pointed out the similarity of the M81 - NGC 3077 bridge to the Magellanic Stream in the Local Group (Mathewson et al. 1974).

M81 and M82 also appear to be joined by a clumpy bridge of HI (Rots 1975, Gottesman & Weliachew 1977). This feature was mapped in more detail by Cottrell (1977), who proposed a gravitational interaction model where M81 and M82 would have had a very close (~ 9 kpc) encounter about 2 × 10⁸ years ago. Some of the curious morphological and kinematical features of M82 and the velocity disturbances of the outer northern part of M81 may also find an explanation in this encounter model.

The HI bridge and spurs found in the NGC 4631 - NGC 4656 system (Roberts 1968, Weliachew 1969, Winter 1975, Weliachew et al. 1978) have been modelled with reasonable success by Combes (1978).

The distorted HI distribution and velocity field in the outer parts of M101 (Beale & Davies 1969, Allen 1975a) may also be caused by gravitational interaction, although a detailed model has yet to be presented.

Byrd (1976) has constructed a model to represent local small disturbances in the velocity field of M31 as the result of a passage of M32 right through the disk of M31. The model does not result in the formation of a large-scale warp of the sort described in Section 4.5 above. The absence of measurable HI in M32 (M_HI < 1.5 × 10⁶ M; Emerson, 1974) presents some difficulty here, in view of the observed features in the M81 - NGC 3077 system.