Some Musings on Galaxy Morphology, Galactic Colors, and the Environments of Galaxies

Careful inspection of large-scale photographs of Shapley-Ames galaxies seems to show a smooth transition between the morphological characteristics of galaxies located on the narrow red and on the broad blue sequences in the galaxian color-magnitude diagram. In other words, there does not appear to be a dichotomy between blue and red galaxies. Both the colors and the morphologies of galaxies are found to correlate strongly with their environments. Red and early-type Shapley-Ames galaxies are dominant in clusters, whereas blue late-type star-forming objects dominate the general field. Interestingly, the colors and morphologies of galaxies in small groups resemble the field and differ from those in clusters. As noted by Baade, the presence of dust and star formation are very closely correlated, except in a few galaxies that probably had unusual evolutionary histories. Over the entire range from S0 to Sc there is no significant difference between the integrated colors of normal and barred objects, suggesting that the formation of a bar does not significantly affect the stellar evolutionary history of a galaxy.