In this paper we have presented new broad-band infrared colors and CO and H2O absorption indices for a number of galaxies distributed in morphological type along the Hubble sequence. The data indicate that a fundamental similarity exists in the nuclear red stellar component of all galaxies dominated optically by a stellar population with mean spectral type ranging from near K to near F. This component appears compatible only with stellar synthesis models characterized by giant branches rich in late M stars, flat main-sequence luminosity functions, and values of M / LV < 10. In only the latest-type galaxies is there perhaps some evidence for a decrease in the influence of late M giants on the 2 µm light. The well-defined morphological distribution that was found for galaxies in the UVK plane should prove particularly useful for future evolutionary star formation studies.
In combination with previously obtained results for ellipticals, we have examined the correlation of the various colors with galaxian luminosity, inclination angle, and projected aperture size. The elliptical colors correlate more strongly than the spiral colors with luminosity, while the reverse is true for inclination. While no radial gradients are found in the spiral sample for the purely infrared colors, both U - V and V - K color gradients appear to be present in all morphological types and generally tend to redden with decreasing aperture size.
Several emission mechanisms have been considered in relation to the small subset of galaxies found to have nuclear 2.2 µm excesses. A warm dust component (with T ~ 700 K) seems likely to be present in at least a few cases. Accurate measurements at L (3.5 µm) would be very useful in this regard. Detailed comparison was made with recently-published synthesis models by Turnrose (1976) and Williams (1976); the latter author's models were found to be incompatible with the infrared data.