A Theory of Changes and the Fundamental Plane

Elliptical galaxies have been observed to cluster near a ribbon along a two-dimensional plane (the Fundamental Plane of Elliptical Galaxies) in the three dimensional space of effective radius, effective surface brightness and central velocity dispersion. This observed clustering holds for galaxies spanning several orders of magnitude in size and total brightness and residing in a wide variety of environments. In order to understand the Fundamental Plane (FP), it is important to know both why it should arise, and how it will evolve. This contribution addresses the second of these, how the FP is likely to change with time. In that context, we consider the evolution due to galaxy-galaxy interactions, starting with arguments based upon the virial theorem, and a discussion of the importance of understanding the mass-to-light ratio as a function of position in the galaxies. The basic result of this argument is that most of any global change in a galaxy due to a transformation (of whatever cause) is mostly projected along the Fundamental Plane. A secondary point is that if the change includes a substantial change in the mass-to-light ratio, a galaxy can move across the FP, which could help explain the differences in observational properties between the regular and dwarf galaxies.