THE RELATION BETWEEN THE VIRIAL-THEOREM AND THE FUNDAMENTAL PLANE OF ELLIPTIC GALAXIES

The analysis of the properties of a sample of elliptical galaxies for which the kinetic energy of both random and rotational motions are con sistently defined, shows that more than half of the so-called 'tilt' o f the fundamental plane of elliptical galaxies is accounted for by the non-homology in the dynamical structure of the systems. We find that the kinetic energy of random motions is proportional to sigma(0)(1.6) instead of sigma(0)(2). We also confirm that some fraction of the tilt is accounted for by the rotation. The remaining 30% of the tilt, on t he other hand, can be easily explained by stellar population effects a nd, possibly, by spatial non-homology. We find that, in the present sa mple, there is no correlation between mass-to-light ratio and luminosi ty. We are thus led to conclude that the existence of the fundamental plane can be consistently explained by the virial theorem, or, in othe r words, that elliptical galaxies are non-homologous virialized system s. The presence of the dynamical non-homology casts some doubts on the use of the FP as a distance indicator.