MERGERS OF DISSIPATIONLESS SYSTEMS - CLUES ABOUT THE FUNDAMENTAL PLANE

We report the results of a set of simulations that were designed to study the physical properties of the ''fundamental plane'' (FP) of elliptical galaxies. By starting with two similar king models and varying the orbital energy and angular momentum, we have investigated the global properties of the remnants. The characteristic parameters of the end products seem to follow closely the observed FP. This trend was confirmed by a subsequent set of simulations in which merger remnants were merged among themselves; that is, merging of objects in the FP produces a new object in the FP. We find that the small departure from the virial theorem, which is characteristic of the fundamental plane correlations, is explained by the nonhomologous nature of the remnants. The simulated FP has a very small intrinsic scatter: compared to the observed 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 observed fundamental planes. Even though merging is selective (not all cosmological reasonable orbits merge), it cannot completely erase initial conditions. In other words, given appropriately correlated primordial units, subsequent merging will maintain a fundamental plane. Therefore, under the ''dissipationless'' approximation here, the FP should be independent of redshift (except for the evolution of their stellar populations) even if substantial merging has occurred.