Colors of galaxies with continuing star formation

The position of nonelliptical galaxies on a two-color (B-V)-(U-B) diagram is examined on the basis of RC2 catalog data, with correction made for interstellar reddening. The sequence of color indices on the two-color diagram is compared with theoretical sequences constructed for Salpeter's initial mass function (IMF) of stars; it is required in the present analysis that the IMF change systematically along a morphological Hubble's sequence of galaxies. The relation between the dust content of galaxies and their IMF is found to be in qualitative agreement with Kahn's theory (1974), which gives an upper limit to the mass of young stars.