THE FUNDAMENTAL PLANE OF GALAXY CLUSTERS

The velocity dispersion sigma, radius R and luminosity L of elliptical galaxies are known to be related, leaving only two degrees of freedom and defining the so-called 'fundamental plane'. In this Letter, we present observational evidence that rich galaxy clusters exhibit a similar behaviour. Assuming a relation L is-proportional-to R(alpha) sigma2beta, the best-fitting values of alpha and beta are very close to those defined by galaxies. The dispersion of this relation is lower than 10 per cent, i.e. significantly smaller than the dispersion observed in the L-sigma and L-R relations. We briefly discuss some possible implications for the spread of formation times of objects and for peculiar velocities of galaxy clusters.