Abstract

We combine Galaxy Evolution Explorer GALEX near-ultraviolet (NUV) photometry with a volume-limited sample of local (0.005 z 0.037) Sloan Digital Sky Survey (SDSS) DR4 galaxies to examine the composition and the environmental dependencies of the optical and ultraviolet (UV)-optical colour-magnitude (CM) diagrams. We find that similar to 30 per cent of red-sequence galaxies in the optical CM diagram show signs of ongoing star formation from their spectra having EW(H alpha) > 2 angstrom. This contamination is greatest at faint magnitudes (M-r > -19) and in field regions where as many as three-quarters of red-sequence galaxies are star forming, and as such has important consequences for following the build-up of the red sequence. The NUV - r colour instead allows a much more robust separation of passively evolving and star-forming galaxies, which allows the build-up of the UV-selected red sequence with redshift and environment to be directly interpreted in terms of the assembly of stellar mass in passively evolving galaxies. In isolated field regions, the number density of UV-optical red-sequence galaxies declines rapidly at magnitudes fainter than M-r similar to -19 and appears completely truncated at M-r similar to -18. These results support the downsizing paradigm whereby the red sequence is assembled from the top down, being already largely in place at the bright end by z similar to 1, and the faint end filled in at later epochs in clusters and groups through environment-related processes such as ram-pressure stripping or galaxy harassment.