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>Mg>−22) and cluster-centric radii (0<rcluster< 1.3 r200). We present the 2D bulge-disc decomposition of galaxies in deep Canada-France-Hawaii Tele-scope u, g, i imaging using GALFIT. Rigorous filtering is applied to identify an analysis sam-ple 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 compo-nent sizes to vary between bands. Gradients are required for 30 % of analysis sample galax-ies. Bulge half-light radii are found to be uncorrelated with galaxy luminosity (Re ∼ 1 kpc, n ∼ 2) for all but the brightest galaxies (Mg < −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 × older, or ∼ 2 × more metal-rich than discs. The