Photometric studies of composite stellar systems. IV - Infrared photometry of globular clusters in M31 and a comparison with early-type galaxies

Abstract

The results of an infrared photometric investigation of 40 globular clusters in and around M31 are presented. The (V - K)_0 colors of the M31 globulars are tightly correlated with other broadband colors and with reddening-free metallicity parameters derived from optical spectrophotometry by Searle. Over a range of ˜1.2 mag in (V - K)_0, the scatter is consistent with observational error. Thus the 0.3-2.2 μm energy distributions are uniquely predicted by the metallicity and vice versa. A comparison of the (V - K)0 colors with those of galactic globulars allows an independent derivation of the metallicities of individual M31 globulars. The broad-band infrared data are compared with predictions from integrated light models based on the Ciardullo and Demarque isochrones. The agreement is quite good for models with an initial mass function of slope ≲ the Salpeter value independent of metallicity, thus ruling out the possibility that a late-type dwarf component is making a significant contribution to the infrared light. CO and H_2O indices measured for eight and seven of the clusters, respectively, give the same result. Early-type galaxies are seen to have much redder broad-band colors and stronger CO and H_2O indices than the most metal-rich M31 or galactic globular observed. Compared to the reddest globular clusters, at a given (U - V)_0 early-type galaxies are on average 0.3 mag redder in (V - K)_0. Although the stellar synthesis models reproduce cluster broad-band colors reasonably well, they do not reproduce the U - V/ V - K distribution of early-type galaxies. We propose that the early-type galaxies contain a population of cool luminous stars present neither in the clusters nor in the stellar synthesis models. One candidate for this population is a giant branch of stars considerably more metal rich than the Sun. More interesting is the possibility that there is a contribution to the integrated infrared light from asymptotic giant-branch stars above the first red giant tip. Such stars could be of intermediate age. The luminosity functions for the M31 and the galactic globulars are examined with the aid of models to investigate the possibility that metal-enhanced star formation or variations in the initial mass function can be detected in integrated light. Two appendices present new infrared data for a faint dE galaxy in the Virgo cluster, and a recalibration of the integrated light models presented by Aaron son et al..

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