Calibration of metallicity effects on the integrated colors of globular clusters and early-type galaxies

IR observations of 42 globular clusters are presented which, in combination with broadband optical colors, provide a strictly empirical calibration of integrated color change with metallicity. Population models with varying metallicity and initial mass function (IMF) are constructed from theoretical isochrones in order to obtain a theoretical fit to the globular-cluster data. The model results are then compared with observations of early-type galaxies to determine the most likely values of metallicity and the parameter s (s = 1 + x, where x is the slope of the IMF) in such galaxies. It is found that: (1) there is a strong correlation between the metallicity of globular clusters and their integrated IR colors; (2) the population models adequately reproduce empirical colors for Z = 0.01; and (3) the location of the centers of early-type galaxies in two-color diagrams forms an extension of the globular-cluster sequence and suggests that galaxies ordered by absolute magnitude are also ordered by metallicity, with the brightest ellipticals being metal-rich.