Dissecting the red sequence: the bulge and disc colours of early-type galaxies in the Coma cluster

We explore the internal structure of red-sequence galaxies in the Coma cluster across a wide range of luminosities (-17 > M-g > -22) and cluster-centric radii (0 < r(cluster) < 1.3 r(200)). We present the 2D bulge-disc decomposition of galaxies in deep Canada-France-Hawaii Telescope u, g, i imaging using galfit. Rigorous filtering is applied to identify an analysis sample of 200 galaxies which are well described by an 'archetypal' S0 structure (central bulge + outer disc). We consider internal bulge and/or disc colour gradients by allowing component sizes to vary between bands. Gradients are required for 30 per cent of analysis sample galaxies. Bulge half-light radii are found to be uncorrelated with galaxy luminosity (R-e similar to 1 kpc, n similar to 2) for all but the brightest galaxies (M-g < -20.5). The S0 discs are brighter (at fixed size, or smaller at fixed luminosity) than those of star-forming spirals. A similar colour-magnitude relation is found for both bulges and discs. The global red sequence for S0s in Coma hence results from a combination of both component trends. We measure an average bulge - disc colour difference of 0.09 +/- 0.01 mag in g - i, and 0.16 +/- 0.01 mag in u - g. Using simple stellar population models, bulges are either similar to 2-3 times older, or similar to 2 times more metal rich than discs. The trend towards bluer global S0 colours observed further from Coma's core is driven by a significant correlation in disc colour with cluster-centric radius. An equivalent trend is detected in bulge colours at a marginal significance level. Our results therefore favour environment-mediated mechanisms of disc fading as the dominant factor in S0 formation.