Tidal interactions in the expanding Universe: the formation of prolate systems

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Abstract
We investigate the possibility that the tendency of galaxies and clusters of galaxies to be slowly rotating but aspherical could be due to early tidal distortion by neighbouring protostructures. Distortions of the right order could have been induced between recombination and the present epoch. We argue that these distortions should not be associated with a great deal of angular momentum: during the linear stage of the development of protostructures, anisotropy energy is a growing first-order quantity, whereas angular momentum grows only in second order and, during the non-linear stage, the alignments of tidally-induced anisotropy and the torquing tidal field are unfavourable to the acquisition of angular momentum. We suggest an observational test for the correlated elongations of neighbouring clusters of galaxies that are predicted by this model. The model suggests that ellipticity should decline with increasing mass.

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