New D -   results for Coma cluster ellipticals

We present velocity dispersions (σ) and photometric diameters (D) for 51 ellipticals in the Coma cluster. Unlike previous studies, our sample includes many ellipticals that lie well outside the cluster core. These data have been used to estimate the environmental dependence of the D – σ relation. The zero-point of the D – σ relation is found to vary by only 6 ± 9 per cent from Coma's core to a projected distance of ~ 4 h – 1 Mpc. This is in the sense that the outer ellipticals possess, on average, slightly larger D diameters at a fixed σ. The projected galaxian density from the core to our outermost ellipticals decreases by a factor of over 150. Thus over a very wide range of galaxian densities we find no evidence for a significant D – σ zero-point variation and hence the D – σ relation appears to be a remarkably robust distance indicator. However, we emphasize that the statistical uncertainties of this statement are still relatively large, particularly for peculiar velocity studies. Whereas previous data for Coma cluster ellipticals showed strong evidence for a surface-brightness bias in the D – σ indicator, our sample does not show such a bias. Also, whereas previous work had suggested that fundamental planes defined by the Virgo and Coma cluster ellipticals were intrinsically different, our new sample does not support this claim. These disagreements may be due to a real intrinsic difference between the core and halo ellipticals of Coma. Unfortunately, the present data are insufficient to address this possibility adequately. However, we do find that the ellipticals inhabiting Coma's core possess a significantly smaller scatter about the fundamental plane than those that lie in the outer parts. We demonstrate that photometric diameters defined at a level one magnitude fainter than the usual level are well correlated with σ and the relationship possesses only a marginally larger scatter than the normal D – σ relation.