In recent years many programs have been developed for recovering information about the stellar content of galaxies from their composite light. Such programs usually follow one of two approaches: In empirical models, some mathematical technique is used to find the best fitting mix of stars which matches detailed spectrophotometric or spectroscopic data; while in evolutionary models, an assumed mass function and star formation rate is used to evolve stars along theoretical tracks from which one can then compute integrated colors to compare with observations. In either case, published synthesis work has been based largely on optical data alone. From such data one can learn very little about the presence and nature of the coolest stars since these emit radiation mostly longward of optical wavelengths.
Knowledge of the coolest stellar component in galaxies would set strong constraints on both empirical and evolutionary synthesis models, and would lead to new insight into the relation between systems with different star formation histories. Consequently, I have undertaken a study of the infrared radiation from ordinary galaxies for the purposes of relating such information to the probable components of the stellar population in these systems. That such a study is now possible is due primarily to the recent development of InSb detectors highly sensitive in the 1 - 5 µm region.
The observations presented here consist largely of 1) broad-band magnitude measurements at 1.2 µm, 1.6 µm, and 2.2 µm; and 2) measurements of the 2.3 µm CO absorption band and 1.9 µm H2O absorption band, features which are sensitive measures of luminosity and effective temperature in late-type stars. Because this study is in some sense a first attempt to obtain infrared photometric data of a quality comparable with that in the optical (i.e., having photometric errors < 0.05 mag), a great deal of attention has been paid to identifying sources of systematic error and making accurate estimates of the true precision obtained.
An endeavor will be made to address a number of questions: giant stars or dwarf stars dominate the 2 µm light? How late in mean spectral type are these stars, and in particular, can limits be placed on the very latest-type stars such as carbons or Miras? Do the infrared colors vary with galaxian luminosity? Do the colors vary with projected aperture size? Does the red stellar component change with galaxian morphology? Are color variations which may be found due primarily to a "pure" population change or to a metallicity-driven population change? Can the interrelations between the various infrared and optical colors be reasonably interpreted? Are there galaxies with non-stellar excesses at 2 µm and if so what are the likely causes of such excesses? How well do the observations support published synthesis models?
The results of this study are presented in Chapters II - V, in the form of four publishable papers. Chapters II and III, which are primarily concerned with the composite light of elliptical galaxies, were written in very close collaboration with Jay Frogel and Eric Persson. The measurements in these papers were obtained mostly with equipment at the Hale Observatories.
The main body of the thesis is Chapter IV, where observations of galaxies with a wide range of morphology are presented and discussed in relation to the data in the previous chapters. In order to obtain the measurements in this paper, I undertook the construction of an InSb detector system, some details of which are given at the end of Chapter IV in Appendix B. It should be noted that a major portion of the work in this thesis went into the building and maintenance of said instrument, which, thank God, worked quite well.
The final paper, in Chapter V, grew out of observations originally obtained for the previous chapter. Although the paper is somewhat tangential to the main themes in the thesis, the results are sufficiently interesting to warrant inclusion anyway.
Chapters II and III have already been submitted to the Ap. J. and are in press. A condensed version of Chapter IV will be submitted to the Ap. J. Suppl., and Chapter V will be sent to the P.A.S.P. Note that in the text, Chapters II, III, and IV are referred to as Papers I, II, and IV.