The Millennium Galaxy Catalogue: Exploring the Color-Concentration Bimodality via Bulge-Disc Decomposition

Abstract

We investigate the origin of the galaxy color-concentration bimodality at the bright-end of the luminosity function (M(B) - 5 log h < -18 mag) with regard to the bulge-disc nature of galaxies. Via (2D) surface brightness profile 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 10^8 h Msol 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 10^8 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 \pm 0.1 x 10^-4 h^3 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).