Our preliminary study of globular clusters is based on the integrated light data presented in Table 2. The colors are compared with those of the galaxies in Figures 3 - 5. M15 and M92 are examples of stellar systems with extreme metal deficiencies, M3 and M13 of moderate metal deficiencies, and M69 of only a small metal deficiency (Sandage 1970; Hartwick and Sandage 1968, and references therein). It is apparent from the color - color plots (Figs. 4 and 5) that these five clusters extend the sequences defined by the E and S0 galaxies and, in fact, overlap the two faintest galaxies NGC 205 and 404. This behavior is similar to that found from studies of optical colors and indices (e.g., McClure and van den Bergh 1968; Faber 1973b). Furthermore, the ordering of these clusters by their infrared colors corresponds approximately to an ordering by metallicity in the sense that redder colors are associated with higher (Fe/H). 4 This result differs from the finding of Grasdalen (1974), who derived V - K colors for a number of highly reddened distant clusters.
The location of the clusters relative to the galaxies on plots of CO versus color (Fig. 3) may differ from the relative locations on color-color plots. There is evidence that for a given color, the CO strengths in globular clusters are weaker than would be estimated from a linear extrapolation of-the available galaxy data. Alternatively, the possibility exists that the CO index for galaxies turns sharply downward below V - K = 3.0.
4 Observations of individual stars in clusters (Cohen, Frogel, and Persson 1978) in fact shows that for a constant effective temperature the CO index decreases with decreasing metallicity. Back.