ON THE FORMATION OF COMPACT ELLIPTICALS

A model for the formation of compact elliptical galaxies as satellites of massive galaxies is presented. It is assumed that compact ellipticals formed through a starburst and the subsequent violent collapse of the stellar system. Numerical N-body experiments show that one can understand the peculiar structure of compacts if they were produced as a result of the relaxation of initially cold stellar systems which were disturbed by the tidal field of a bright galaxy. The observed differences between compact ellipticals and low-mass giant ellipticals might therefore result from the fact that the compacts formed within the potential well of another galaxy whereas the giant ellipticals evolved as isolated systems. Constraints on the internal rotation and on the orientation of the major and minor axes with respect to the orbital plane of the compacts are derived. These results can be used to deproject the observed surface brightness and velocity profiles of compact ellipticals, and to calculate their orbital parameters. Additional N-body experiments of collapsing, initially clumpy protogalaxies indicate that compact ellipticals might be the remnants of those clumps that did not merge, but gained energy and angular momentum and formed separate satellite systems. Thus the compacts might provide important information on the building blocks of giant galaxies as well as on the physical conditions of the protogalactic environment in which they evolved.