The development of InSb detectors highly sensitive in the 1 - 5 µm region has finally permitted detailed photometric study of the red stellar population in external galaxies. This thesis is the result of such an investigation. we have shown that the red stellar component appears to be fundamentally similar in the central regions of bright galaxies of all but perhaps the latest morphological types, we have learned that this component is compatible only with stellar synthesis models having small mass-to-light ratios - models that are rich in late M giants, but which have only a minor contribution from late M dwarfs. The relation of infrared and optical colors between ellipticals and globular clusters was found to provide further support for the hypothesis that the color-luminosity distribution in early type galaxies is a metallicity sequence. on the other hand, the U - V, V - K color-morphology distribution that was identified here for the first time could be well reproduced by a simple population model.
A number of interesting problems remain: Detailed empirical or evolutionary models which fully incorporate the infrared data are certainly warranted. Evolutionary models coupled with additional infrared observations of very blue galaxies may clarify the question of whether there. are truly young galaxies, or just old galaxies with recent star formation bursts. A survey of UVK colors in galaxy clusters of varying richness might reveal interesting differences in star formation history between such clusters. Detailed point mapping of CO and H2O indices off the nucleus might better indicate whether broad band galaxian color, gradients are primarily population or metallicity driven. Assuming the latter is the case in early-type galaxies, whether our knowledge of giant branch evolution can properly account for either the V - K color-aperture or color-luminosity relations discovered in this study is presently unclear. Finally, exploitation of new infrared spectral features which, could better pin dawn the nature of the very latest M stars in galaxies would be extremely useful.