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Attention has recently been drawn to the existence of sharply defined, shell-like, optical features in the outer regions of many elliptical galaxies. Simple N-body models have been constructed which show these features to be consistent with structures formed by the collision of an elliptical galaxy and a disk galaxy. The shells arise from a phase wrapping of the dynamically cold disk in the fixed potential well of the elliptical galaxy. The test particle dynamics of the shells, and the large range in radius they cover, may allow them to be used to develop an impression of the detailed shape of potential wells in elliptical galaxies. The shell structures around several ellipticals suggest that they have a massive dark component, but these components do not extend to large radii and they do not have an isothermal-like mass distribution. The distribution and structure of shell galaxies will afford a new opportunity to examine the importance of galaxy interactions for galaxy formation and evolution.
Peter J. Quinn (Sun,) studied this question.