The fundamental plane of cluster ellipticals

We show that, for the Coma cluster ellipticals of normal surface brightness, there is a highly significant correlation between the residual from the Dn – σ relation and effective surface brightness µe( B ). The higher surface brightness ellipticals give systematically larger Dn – σ distances. New, independent V-band photometry, when combined with the published velocity dispersion data, also show the same, highly significant correlation. This correlation arises because, for the Coma ellipticals, Dn does not give a precise edge-on view of the fundamental plane. The Coma ellipticals with µe(B) < 22.0 define a fundamental plane with no intrinsic scatter, indicating a remarkable degree of homogeneity. We show that there is very suggestive evidence that the elliptical galaxies of the Virgo and Coma clusters lie on intrinsically different fundamental planes. This difference limits the accuracy that the relative Virgo–Coma distance can be determined from techniques based on the fundamental plane to ~ 9 per cent.