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 ∼ 1 kpc, n ∼ 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 ∼2-3 times older, or ∼ 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.