Variable mass structure and the Fundamental Plane

We examine the influence of broken structural homology upon the Fundamental Plane (FP). We fit the Sersic R1/n law, being the generalized R1/4 law, where 'n' is a free parameter that accommodates structural differences between different galaxies. The galaxy light profiles show a trend of systematic departures from the de Vaucouleurs R1/4 law, such that the larger galaxies have less curvature in their profiles than the R1/4 profile and the smaller galaxies have greater curvature, as found by Caon, Capaccioli & D'Onofrio (1993) and Graham et al. (1996). This results in the effective half-light radii, Re, and the mean surface brightness within these radii, having systematic biases if obtained from the R1/4 law. The observed range in structural shapes implies a corresponding range in galaxy dynamics required to support the observed galaxy structure. Allowing for this, we find that broken structural and dynamical homology are partly responsible for the 'tilt' of the FP.