Curvature in the colour-magnitude relation but not in colour-Σ: major dry mergers at M* > 2 x 1011 M⊙?

The colour-magnitude relation of early-type galaxies differs slightly but significantly from a pure power law, curving downwards at low and upwards at high luminosities (M-r > -20.5 and M-r < -22.5, respectively). This remains true of the colour-size relation, and is even more apparent with stellar mass (M-* < 3 x 1010 M-circle dot and M-* > 2 x 1011 M-circle dot, respectively). The upward curvature at the massive end does not appear to be due to stellar population effects. In contrast, the colour-Sigma relation is well described by a single power law. Since major dry mergers change neither the colours nor Sigma, but they do change masses and sizes, the clear features observed in the scaling relations with M-*, but not with Sigma > 150 km s-1, suggest that M-* > 2 x 1011 M-circle dot is the scale above which major mergers dominate the assembly history. We discuss three models of the merger histories since z similar to 1 which are compatible with our measurements. In all three models, dry mergers are responsible for the flattening of the colour-M-* relation at M-* > 3 x 1010 M-circle dot - wet mergers only matter at smaller masses. At M-* > 2 x 1011 M-circle dot, the merger histories in one model are dominated by major rather than minor dry mergers. In another, although both major and minor mergers occur at the high-mass end, the minor mergers contribute primarily to the formation of the intracluster light (ICL), rather than to the stellar mass growth of the central massive galaxy. This model attributes the fact that alpha < 1, in the scaling M-* proportional to M alpha(dyn), to the formation of the ICL. A final model assumes that the bluest objects today were assembled by minor dry mergers of the bluest (early-type) objects at high redshift, whereas the reddest objects were assembled by a mix of major and minor dry mergers. In this model, the scatter of the colour-magnitude relation should increase with redshift, and the dependence on environment should also be more pronounced at higher redshift: more clustered objects should be redder. Similar measurements of these relations at high redshift will provide further valuable constraints on the mass scale at which major dry mergers dominate the assembly history.