  We explore the origin of the colour-magnitude relation (CMR) of early type
galaxies in the Virgo cluster using spectra of very high S/N ratio for six
elliptical galaxies selected along the CMR. The data are analysed using a new
evolutionary stellar population synthesis model to generate galaxy spectra at
the resolution given by their velocity dispersions. In particular we use a new
age indicator that is virtually free of the effects of metallicity. We find
that the luminosity weighted mean ages of Virgo ellipticals are greater than ~8
Gyr, and show no clear trend with galaxy luminosity. We also find a positive
correlation of metallicity with luminosity, colour and velocity dispersion. We
conclude that the CMR is driven primarily by a luminosity-metallicity
correlation. However, not all elements increase equally with the total
metallicity and we speculate that the CMR may be driven by both a total
metallicity increase and by a systematic departure from solar abundance ratios
of some elements along the CMR. A full understanding of the role played by the
total metallicity, abundance ratios and age in generating the CMR requires the
analysis of spectra of very high quality, such as those reported here, for a
larger number of galaxies in Virgo and other clusters.
