Interacting and Merging Galaxies The galaxy distribution includes a substantial number of bound systems dominated by two galaxies (hereinafter "pairs"). These simplest systems of galaxies are excellent laboratories for studying galaxy masses (since we understand two-body dynamics better than those in clusters, and since the range of separations in pairs extends much farther than individual galaxy rotation curves), and for study of the effects of perturbations on galaxies. Overall references on this topic include the proceedings of IAU Colloquium 124 and the book Double Galaxiesby Karachentsev (in Russian; there is an English version hosted by NED).Recognizing pairs:There are special difficulties in recognizing complete sets of pairs. Various criteria have been considered, relying on either projected separation or including radial-velocity information. Early works simply used a diameter vs. separation criterion. More recent ones have incorporated isolation criteria, as illustrated in this diagram following Fig. 1 of Karachentsev's
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