Some effects of galaxy structure and dynamics on the Fundamental Plane. II. A Virgo-Fornax distance modulus

The influence of broken structural homology, and the implied broken dynamical homology, is examined for the Fundamental Plane (FP). Requiring a symmetrical treatment of the FP variables, a bisector method of linear regression was applied, in 3-dimensions, to derive the best FP. A bootstrapping procedure has been used to estimate the uncertainties associated with the slope of the FP. For 25 E and S0 Virgo galaxies, the ‘standard’ FP, constructed using R model parameters for the effective radii (Re,4) and the mean surface brightness within this radius (Σe,4) and using central velocity dispersion (CVD) measurements (σ0), gave a relation described by Re,4 ∝ σ 1.10±0.14 0 Σ e,4 . Using Sersic R 1/n light profile model parameters and the projected, infinite aperture, velocity dispersion (σtot,n), derived from application of the Jeans equation to the observed intensity profiles, gave an ‘improved’ FP described by the relation Re,n ∝ σ 1.37±0.16 tot,n Σ −0.76±0.05 e,n . This result, based on independent data, supports the previous finding by Graham & Colless (1997a) that assumptions of structural and dynamical homology are partly responsible for the departure of the observed FP from the plane expected by the virial theorem, which predicts R ∝ σ Σ. Upon removal of the known S0 galaxies from the sample of Virgo galaxies, the above planes were observed to change to Re,4 ∝ σ 1.19±0.21