• galaxies: distances and redshifts;
  • galaxies: photometry;
  • galaxies: stellar content


The bimodal galaxy distribution in the optical colour–magnitude diagram (CMD) comprises a narrow ‘red sequence’ populated mostly by early-type galaxies and a broad ‘blue cloud’ dominated by star-forming systems. Although the optical CMD allows one to select red sequence objects, neither can it be used for galaxy classification without additional observational data such as spectra or high-resolution images, nor to identify blue galaxies at unknown redshifts. We show that adding the near ultraviolet (NUV) colour [Galaxy Evolution Explorer (GALEX) NUV λeff= 227 nm] to the optical (g − r versus Mr) CMD reveals a tight relation in the 3D colour–colour–magnitude space smoothly continuing from the ‘blue cloud’ to the ‘red sequence’. We found that 98 per cent of 225 000 low-redshift (Z < 0.27) galaxies follow a smooth surface inline image with a standard deviation of 0.03–0.07 mag making it the tightest known galaxy photometric relation, given the ∼0.9 mag range of k-corrected g − r colours. Similar relations exist in other NUV–optical colours. There is a strong correlation between morphological types and integrated inline image colours of galaxies, while the connection with g − r is ambiguous. Rare galaxy classes such as E+A or tidally stripped systems become outliers that occupy distinct regions in the 3D parameter space. Using stellar population models for galaxies with different star formation histories, we show that (a) the (inline image) distribution at a given luminosity is formed by objects having constant and exponentially declining star formation rates with different characteristic time-scales with the red sequence part consistent also with simple stellar population; (b) colour evolution for exponentially declining models goes along the relation suggesting a weak evolution of its shape up to a redshift of 0.9; (c) galaxies with truncated star formation histories have very short transition phase offset from the relation thus explaining the rareness of E+A galaxies. This relation can be used as a powerful galaxy classification tool when morphology remains unresolved. Its mathematical consequence is the possibility of precise and simple redshift estimates from only three broad-band photometric points. We show that this simple approach being applied to Sloan Digital Sky Survey and GALEX data works better than most existing photometric redshift techniques applied to multicolour data sets. Therefore, the relation can be used as an efficient search technique for galaxies at intermediate redshifts (0.3 < Z < 0.8) using optical imaging surveys.