Some Effects of Galaxy Structure and Dynamics on the Fundamental Plane

Abstract

We examine the effects on the Fundamental Plane (FP) of structural departures from an R^{1/4} galaxy light profile. We also explore the use of volumetric as well as projected galaxy parameters. We fit the Sersic R^{1/n} law to the V-band light profiles of 26 Virgo ellipticals, where `n' is a shape parameter that allows for structural differences amongst the profiles. The galaxy light profiles show a trend of systematic departures from a de Vaucouleurs R^{1/4} law, in the sense that `n' increases with increasing effective half-light radius R_{e}. Adjustments to the measured velocity dispersion are also made, based upon the theoretical velocity dispersion profile shapes of the different R^{1/n} light profiles. We find that allowing for broken structural homology through fitting R^{1/n} profiles, but still using central velocity dispersions, actually increases the departure of the observed FP from the virial plane - the increase in effective radius with galaxy luminosity (and `n') is over-balanced by an associated decrease in the mean surface brightness. In examining the use of volumetric quantities and allowing for the different velocity dispersion profiles corresponding to the observed light profiles, we find that use of the volumetric velocity dispersion at the volumetric half-light radius decreases the departure of the FP from the virial plane. Through use of the Jeans hydrodynamical equation, we further convert the projected central aperture velocity dispersion, sigma_{0}, into the infinite aperture velocity dispersion, sigma_{tot,n} (which is equal to one-third of the virial velocity dispersion). Using both the R^{1/n} fit parameters and sigma_{tot,n} we obtain for the FP, R_{e,n}~sigma_{tot,n}^{1.44+/-0.11}Sigma_{e,n}^{-0.93+/-0.08}, where R_{e,n} and Sigma_{e,n} are the radius and surface brightness terms.