The millennium galaxy catalogue: Bulge-disc decomposition of 10 095 nearby galaxies

We have modelled the light distribution in 10095 galaxies from the Millennium Galaxy Catalogue (MGC), providing publicly available structural catalogues for a large, representative sample of galaxies in the local Universe. Three different models were used: (1) a single Sérsic function for the whole galaxy, (2) a bulge-disc decomposition model using a de Vaucouleurs (R^1/4) bulge plus exponential disc, (3) a bulge-disc decomposition model using a Sérsic (R^1/n) bulge plus exponential disc. Repeat observations for ~700 galaxies demonstrate that stable measurements can be obtained for object components with a half-light radius comparable to, or larger than, the seeing half width at half-maximum. We show that with careful quality control, robust measurements can be obtained for large samples such as the MGC. We use the catalogues to show that the galaxy colour bimodality is due to the two-component nature of galaxies (i.e. bulges and discs) and not due to two distinct galaxy populations. We conclude that understanding galaxy evolution demands the routine bulge-disc decomposition of the giant galaxy population at all redshifts.