The metallicities, velocity dispersions and true shapes of elliptical galaxies

The relationships between velocity dispersion, line strength and absolute magnitude have been re-analysed for normal elliptical galaxies. It is found that, at fixed absolute magnitude, galaxies with high velocity-dispersions tend to have high line-strengths and those with low velocity-dispersions have low line-strengths. This implies that elliptical galaxies are at least a two-parameter family. This conclusion is based on a preliminary sample of 24 galaxies for which both types of measurements are available. Well-known apparent scale differences in velocity dispersion between various authors are shown to be due to mean line-strength differences between galaxy samples. After correction for this effect, all sources considered here agree well to within a few per cent. The present data also suggest that the velocity-dispersion and line-strength residuals may in turn be correlated with intrinsic axial ratio, with flatter ellipticals having smaller velocity dispersions and metallicities. These new results are discussed in the context of dissipational and dissipationless collapse models for elliptical galaxy formation. They indicate a strong link between the dynamical and chemical evolutions of elliptical galaxies which seems quite compatible with the dissipational picture. (author)