• galaxies: bulges;
  • galaxies: evolution;
  • galaxies: photometry;
  • galaxies: structure


We present a set of bulge–disc decompositions for a sample of 71 825 Sloan Digital Sky Survey (SDSS) main-sample galaxies in the redshift range 0.003 < z < 0.05. We have fitted each galaxy with either a de Vaucouleurs (‘classical’) or an exponential (‘pseudo-’) bulge and an exponential disc. Two-dimensional Sérsic fits are performed when the two-component fits are not statistically significant or when the fits are poor, even in the presence of high signal-to-noise ratio (S/N). We study the robustness of our two-component fits by studying a bright subsample of galaxies and we study the systematics of these fits with decreasing resolution and S/N. Only 30 per cent of our sample have been fitted with two-component fits in which both components are non-zero. The gr and gi colours of each component for the two-component models are determined using linear templates derived from the r-band model. We attempt a physical classification of types of fits into disc galaxies, pseudo-bulges, classical bulges and ellipticals. Our classification of galaxies agrees well with previous large bulge plus disc (B+D) decomposed samples. Using our galaxy classifications, we find that Petrosian concentration is a good indicator of bulge-to-total ratio, while overall Sérsic index is not. Additionally, we find that the majority of green valley galaxies are bulge+disc galaxies. Furthermore, in the transition from green to red B+D galaxies, the total galaxy colour is most strongly correlated with the disc colour.