Color separation of galaxy types in the Sloan Digital Sky Survey imaging data

We study the optical colors of 147,920 galaxies brighter than g* = 21, obse rved in five bands by the Sloan Digital Sky Survey (SDSS) over similar to 1 00 deg(2) of high Galactic latitude sky along the celestial equator. The di stribution of galaxies in the g*-r* versus u*-g* color-color diagram is str ongly bimodal, with an optimal color separator of u*-r* = 2.22. We use visu al morphology and spectral classification of subsamples of 287 and 500 gala xies, respectively, to show that the two peaks correspond roughly to early- (E, S0, and Sa) and late-type (Sb, Sc, and Irr) galaxies, as expected from their different stellar populations. We also find that the colors of galax ies are correlated with their radial profiles, as measured by the concentra tion index and by the likelihoods of exponential and de Vaucouleurs' profil e fits. While it is well known that late-type galaxies are bluer than early -type galaxies, this is the first detection of a local minimum in their col or distribution. In all SDSS bands, the counts versus apparent magnitude re lations for the two color types are significantly different and demonstrate that the fraction of blue galaxies increases toward the faint end.