THE MILLENNIUM GALAXY CATALOGUE: EXPLORING THE COLOR-CONCENTRATION BIMODALITY VIA BULGE-DISK DECOMPOSITION

We investigate the origin of the galaxy color-concentration bimodality at the bright end of the luminosity function (MB - 5log h 70 < -18 mag) with regard to the bulge-disk nature of galaxies. Via (two-dimensional) 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 disk-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 70 M sun 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 70 M sun Mpc-3, respectively). Moreover, luminous, 'bulge-less, red disks', and 'disk-less, blue bulges' (blue ellipticals) are exceptionally rare (with volume densities of (1.7 ± 0.3) and (1.1 ± 0.1) x 10-4 h 370 Mpc-3, respectively). Finally, within the two-component population we confirm a previously reported correlation between bulge and disk color (with a mean offset of only ((u - r)bulge - (u - r)disk) = 0.22 ± 0.02 mag).