3.5 Tidal Tail Extent
In their survey of galactic bridges and tails, Toomre & Toomre (1972) succeeded in producing numerical models of tidal encounters between galaxies which, for a given moment in the evolution sequence, and using the information on the spatial orientation, resembles closely the observed system such a the M51 system, the Antennae (NGC 4038/39), etc. The basic postulate in such models is that gravity is the dominant force (it was frequently thought before that that bridges and tails needed to be explained by invoking magnetic forces).
As a corollary to this work, it was found that for slow encounters between spirals the objects merge to form an elliptical like object, and Toomre (1977) produced a sequence of peculiar galaxies which he considered in various stages of a merging process. This merger hypothesis proves very pervasive, and now forms an integral part of the ``bottom-up'' scenario's of galaxy and structure formation. A full observational study of the sequence of galaxies discussed by Toomre (1977) can be found in Hibbard & Van Gorkom (1996).
Since the study of Toomre & Toomre (1972) was done without dark halos, it was interesting to see whether the addition of halos to N-body simulations would change the story. Barnes (1988) did this for the Antennae (NGC 4038/39), with success. The growing capablities of supercomputers to address the gravitational N-body problem led Dubinski et al. (1996) to reexamine the problem, with the specific intent to delimit the extent of the dark halo. They thus considered models with different halo extent, and found that for large halo to disk mass tail forming is inhibited. Dubinski et al. (1998) extended these calculations, in order to establish a clear criterion to delimit halo extent. However, Barnes (1998) shows that their conclusions are not true for haloes with very shallow density profiles, and thus tidal tail extent may not be a helpful indicator which can be used to rule out very large halos (see also Springel & White 1998).