Galaxy bimodality versus stellar mass and environment

We analyse a z < 0.1 galaxy sample from the Sloan Digital Sky Survey focusing on the variation in the galaxy colour bimodality with stellar mass M and projected neighbour density Sigma, and on measurements of the galaxy stellar mass functions. The characteristic mass increases with environmental density from about 10(10.6) to 10(10.9) M circle dot (Kroupa initial mass function, H-0 = 70) for Sigma in the range 0.1-10 Mpc(-2). The galaxy population naturally divides into a red and blue sequence with the locus of the sequences in colour-mass and colour-concentration indices not varying strongly with environment. The fraction of galaxies on the red sequence is determined in bins of 0.2 in log Sigma and log M (12 x 13 bins). The red fraction f(r) generally increases continuously in both Sigma and M such that there is a unified relation: f(r) = F(Sigma, M). Two simple functions are proposed which provide good fits to the data. These data are compared with analogous quantities in semi-analytical models based on the Millennium N-body simulation: the Bower et al. and Croton et al. models that incorporate active galactic nucleus feedback. Both models predict a strong dependence of the red fraction on stellar mass and environment that is qualitatively similar to the observations. However, a quantitative comparison shows that the Bower et al. model is a significantly better match; this appears to be due to the different treatment of feedback in central galaxies.