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,observed in five bands by the Sloan Digital Sky Survey (SDSS) over ~100 sq.deg. of high Galactic latitude sky along the Celestial Equator. Thedistribution of galaxies in the g*-r* vs. u*-g* color--color diagram isstrongly bimodal, with an optimal color separator of u*-r* = 2.22. We usevisual morphology and spectral classification of subsamples of 287 and 500galaxies respectively, to show that the two peaks correspond roughly to early(E, S0, Sa) and late (Sb, Sc, Irr) type galaxies, as expected from theirdifferent stellar populations. We also find that the colors of galaxies arecorrelated with their radial profiles, as measured by the concentration indexand by the likelihoods of exponential and de Vaucouleurs' profile fits. Whileit is well known that late type galaxies are bluer than early type galaxies,this is the first detection of a local minimum in their color distribution. Inall SDSS bands, the counts vs. apparent magnitude relations for the two colortypes are significantly different, and indicate that the fraction of bluegalaxies increases towards the faint end.