THE FUNDAMENTAL PLANE FOR CLUSTER E AND SO GALAXIES

We have analysed the shape of the Fundamental Plane (FP) for a sample of 226 E and SO galaxies in 10 clusters of galaxies. We find that the distribution of galaxies is well approximated by a plane of the form l og r(e) = 1.24 log sigma - 0.82 log [I](e) + gamma for photometry obta ined in Gunn r. This result is in good agreement with previous determi nations. The FP has a scatter of 0.084 in log r(e). For galaxies with velocity dispersion larger than 100 km s(-1) the scatter is 0.073. If the FP is used for distance determinations this scatter is equivalent to 17 per cent uncertainties on distances to single galaxies. We find that the slope of the FP is not significantly different from cluster t o cluster. Selection effects and measurement errors can introduce bias es in the derived slope. The residuals of the FP correlate weakly with the velocity dispersion and the surface brightness, Some of the coeff icients used in the literature give rather strong correlations between the residuals and absolute magnitudes, This implies that galaxies nee d to be selected in a homogeneous way to avoid biases of derived dista nces on the level of 5-10 per cent or smaller. The FP has significant intrinsic scatter. No other structural parameters like ellipticity or isophotal shape can reduce the scatter significantly. This is in contr adiction to simple models, which predict that the presence of discs in E and SO galaxies can introduce scatter in the FP. It remains unknown what the source of scatter is, It is therefore unknown whether this s ource produces systematic errors in distance determinations. The Mg-2- sigma relation for the cluster galaxies differs slightly from cluster to cluster, Galaxies in clusters with lower velocity dispersions have systematically lower Mg-2. The effect can be caused by both age and me tallicity variations, With the current stellar population models, the best agreement with our results regarding the FP is if the offsets are mainly caused by differences in metallicity, Most of the distances th at we derive from the FP imply small peculiar motions (< 1000 km s(-1) ). The zero point of the FP must therefore be quite stable. Only for o ne cluster, located 28 degrees from the direction towards the 'Great A ttractor', do we find a peculiar motion of 1300 km s(-1). This motion is reduced to 890 km s(-1) if we use the FP corrected for the offset o f the Mg-2-sigma relation, This confirms earlier suggestions that the residuals from the Mg-2-alpha relation can be used to flag galaxies wi th deviant populations, and possibly to correct the distance determina tions for the deviations.