THE FUNDAMENTAL PLANE OF ELLIPTIC GALAXIES AND THE VIRIAL-THEOREM

When an elliptical galaxy undergoes a transformation that satisfies tw o simple physical assumptions, virial equilibrium and retention of the functional form of the surface brightness and mass density profiles, the resulting observable properties of the galaxy lie in a very thin p arallelepiped in the three-dimensional space defined by effective radi us, effective surface brightness, and central velocity dispersion, eve n for large changes in the global mass-to-light ratio (M/L). Additiona lly, for a fixed M/L, they are quite narrowly constrained in the face of the parallelepiped. The angle between this plane and the observed F undamental Plane of elliptical galaxies is less than 15 degrees, imply ing that most of any predicted change is projected along the Fundament al Plane, regardless of the specific details of the interaction respon sible for the change. Tidal interactions are examined as a simple appl ication.