We use the C _ { 2 } 4668 , Fe 4383 , 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 with 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 .
We additionally show evidence for a relation between age and luminosity , in the direction predicted by the semi–analytic hierarchical clustering models of Kauffmann & Charlot [ Kauffmann & Charlot 1998 ] , but this is only present in the C _ { 2 } 4668 index models , and could be an effect of the lack of non solar abundance ratios in the Worthey models used .

By comparing deviations from the CMR , with deviations in absorption index from analogous ‘ index–magnitude ’ relations , we find that colour deviations bluewards of the mean relation are strongly correlated with the hydrogen Balmer line series absorption .
We show that the properties of these blue galaxies are consistent with the presence of a young stellar population in the galaxies , rather than with a reduced metallicity .