Colour-magnitude relations and spectral line strengths in the Coma cluster

We use the C(2)4668, Fe4383, H gamma(A) and H delta(A) spectral absorption line indices, together with U- and V-band photometry of 101 galaxies in the Coma cluster, to investigate how mean age and metal abundance correlate wi th galaxy luminosity. In particular, we use the line index measurements to address the origin of the colour-magnitude relation (CMR). We find that the CMR in Coma is driven primarily by a luminosity-metallicity correlation. W e additionally show evidence for a relation between age and luminosity, in the direction predicted by the semi-analytic hierarchical clustering models of Kauffmann & Charlot, but this is only present in the C(2)4668 index mod els, and could be an effect of the lack of non-solar abundance ratios in th e Worthey models used. By comparing deviations from the CMR with deviations in absorption index fr om analogous 'index-magnitude' relations, we find that colour deviations bl uewards of the mean relation are strongly correlated with the hydrogen Balm er line series absorption. We show that the properties of these blue galaxi es are consistent with the presence of a young stellar population in the ga laxies, rather than with a reduced metallicity.