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The composite nature of the integrated spectra of elliptical galaxies and the domination of Thereplace string visible light by late-type giant stars have been known for nearly 30 years (e. g., Stebbins and Whitford 1948; Morgan and Mayall 1957; Code 1959; reviewed by Whitford 1976). Nevertheless, the lack of luminosity-sensitive indices, particularly in the red and infrared regions of the spectrum, has hindered attempts to build quantitative models of the stellar content of these galaxies. For example, one of the conclusions of a study by Faber (1972), based in part on the observations of Spinrad and Taylor (1971), was that the available optical data could not be used to uniquely determine the proportion of high- and low luminosity red stars. The first broad-band infrared observations of galaxies (Johnson 1966a) showed that the 1.2 - 3.5 µm radiation is contributed mostly by stars cooler than those which dominate the visible light. However, M dwarfs and M giants have similar colors in the infrared, so that Johnson's data were not adequate to determine the relative contributions of the high and low luminosity populations.

Advances in the development of red and infrared sensitive detectors have finally permitted the accurate measurements of both luminosity and temperature-sensitive indices in these spectral regions. Examples of such observations are those of the Ca II + TiO features by O'Connell (1974, 1976a, b), the 2.3 µm CO band by Baldwin et al. (1973a) and Frogel et al. (1975b), and the 9910 Å Wing-Ford band by Whitford (1977). These data are necessary to model the stellar content of elliptical galaxies, and hence to estimate the evolutionary correction to q0 in the redshift-magnitude diagram (Sandage 1961; Faber 1973b; Tinsley 1973, 1975; Tinsley and Gunn 1976; O'Connell 1976b.

In this paper we present new multiaperture observations of the strength of the CO feature in 51 elliptical and lenticular galaxies. Together with these measurements, we have also obtained magnitudes in the 1.2 µm, 1.6 µm, and 2.2 µm photometric bands (J, H, and K) for the same galaxies. These data are combined with U and V observations from the literature to examine variations in the relationships between optical and infrared colors from galaxy to galaxy and within individual galaxies. In Section II, the photometric system, observational procedures, and data reduction are described in some detail. The results of the observations are presented in Sections III and IV. In Sections V and VI, the results are compared with published stellar synthesis models and with other photometric studies of galaxies.

Our conclusions are summarized in Section VII. The system of standard magnitudes and colors, and mean photometric relationships for a selection of red giant and dwarf stars are presented in the Appendix.

Paper II of this series (Aaronson, Frogel and Persson 1977) presents observations of the 2.0 µm H2O absorption band (Baldwin et al. 1973b), which impose additional constrains on the coolest stellar component of the galaxies under investigation. Paper III (Person, Frogel, and Aaronson 1978) presents new multiaperture UBVR and JHK data for nearly 100 field ellipticals and JHK data alone for galaxies in the Virgo and Coma clusters. These data will allow a more thorough investigation of the color-color and color-magnitude relations discussed here.

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