Authors: Ewan Cameron, Simon P. Driver, Alister W. Graham, Jochen Liske.
Abstract: We investigate the origin of the galaxy color-concentration bimodality at the bright-end of the luminosity function (MB - 5 log h < -18 mag) with regard to the bulge-disc nature of galaxies. Via two-dimensional surface brightness modeling with GIM2D, we subdivide the local galaxy population in the Millennium Galaxy Catalogue into one-component and two-component systems. We reveal that one-component (elliptical and disc-only) systems define the two peaks of the galaxy color-concentration distribution, with total stellar mass densities of (0.7±0.1) and (1.3±0.1) x 108 h MSun Mpc-3 respectively, while two-component systems contribute to both a bridging population and the red, concentrated peak, with total stellar mass densities of (1.1±0.1) and (1.8±0.2) x 108 h MSun Mpc-3 respectively. Moreover, luminous, `bulge-less, red discs' and `disc-less, blue bulges' (blue ellipticals) are exceptionally rare, with volume-densities of (1.7±0.3) and (1.1±0.1) x 10-4 h3 Mpc-3 respectively. Finally, within the two-component population we confirm a previously-reported correlation between bulge and disc color, with a mean offset of only < (u-r)bulge - (u-r)disc > = 0.22±0.02 mag.