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

TL;DR

This study analyzes the origin of galaxy color and concentration bimodality by decomposing galaxy structures into bulge and disc components, revealing distinct contributions of different galaxy types to the bimodal distribution.

Contribution

It provides a detailed analysis of the bulge-disc structures in galaxies and their relation to the observed color-concentration bimodality, using 2D surface brightness modeling.

Findings

One-component systems define the two peaks of the bimodality.

Two-component systems contribute to the bridging population and the red, concentrated peak.

Luminous, bulge-less red discs and blue bulges are very rare.

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).