MERGERS OF DISSIPATIONLESS SYSTEMS - CLUES ABOUT THE FUNDAMENTAL PLANE

We report the results of a set of simulations that were designed to st udy the physical properties of the ''fundamental plane'' (FP) of ellip tical galaxies. By starting with two similar king models and varying t he orbital energy and angular momentum, we have investigated the globa l properties of the remnants. The characteristic parameters of the end products seem to follow closely the observed FP. This trend was confi rmed by a subsequent set of simulations in which merger remnants were merged among themselves; that is, merging of objects in the FP produce s a new object in the FP. We find that the small departure from the vi rial theorem, which is characteristic of the fundamental plane correla tions, is explained by the nonhomologous nature of the remnants. The s imulated FP has a very small intrinsic scatter: compared to the observ ed one it enables us to establish that the central potential W-0, and the mass of the primordial units from which galaxies were formed could not have very broad distributions, although this is not a unique way to explain the difference in scatter between the predicted and the obs erved fundamental planes. Even though merging is selective (not all co smological reasonable orbits merge), it cannot completely erase initia l conditions. In other words, given appropriately correlated primordia l units, subsequent merging will maintain a fundamental plane. Therefo re, under the ''dissipationless'' approximation here, the FP should be independent of redshift (except for the evolution of their stellar po pulations) even if substantial merging has occurred.