Near-infrared surface brightness fluctuations and optical colours of Magellanic star clusters

This work continues our efforts to calibrate model surface brightness fluctuation luminosities for the study of unresolved stellar populations, through a comparison with the data of Magellanic Cloud star clusters. We present here the relation between absolute Ks-band fluctuation magnitude and (V-I) integrated colour, using data from the Two-Micron All-Sky Survey (2MASS) and the Deep Near-Infrared Southern Sky Survey (DENIS), and from the literature. We compare the star cluster sample with the sample of early-type galaxies and spiral bulges studied by Liu et al. We find that intermediate-age to old star clusters lie along a linear correlation with the same slope, within the errors, of that defined by the galaxies in the (M) over bar (Ks) versus (V-I) diagram. While the calibration by Liu et al. was determined in the colour range 1.05 < (V-I-C)(0) < 1.25, ours holds in the interval -5 greater than or similar to (M) over bar (Ks) greater than or similar to -9, 0.3 less than or similar to (V-I) less than or similar to 1.25 This implies, according to Bruzual-Charlot and Mouhcine-Lan, con models, that the star clusters and the latest star formation bursts in the galaxies and bulges constitute an age sequence. At the same time, a slight offset between the galaxies and the star clusters [the latter are similar to 0.7 mag fainter than the former at a given value of (V-I)], caused by the difference in metallicity of roughly a factor of 2, confirms that the (M) over bar (Ks) versus (V-I) plane may contribute to break the age-metallicity degeneracy in intermediate-age and old stellar populations. The confrontation between models and galaxy data also suggests that galaxies with K-S fluctuation magnitudes that are brighter than predicted, given their (V-I) colour, might be explained in part by longer lifetimes of thermally pulsing asymptotic giant branch stars. A preliminary comparison between the H 2MASS data of the Magellanic star clusters and the sample of 47 early-type galaxies and spiral bulges observed by Jensen et al. through the F160W Hubble Space Telescope filter leads to the same basic conclusions: galaxies and star clusters lie along correlations with the same slope, and there is a slight offset between the star cluster sample and the galaxies, caused by their different metallicities. Magellanic star clusters are single populations, while galaxies are composite stellar systems; moreover, the objects analysed live in different environments. Therefore, our findings mean that the relationship between fluctuation magnitudes in the near-infrared, and (V-I) might be a fairly robust tool for the study of stellar population ages and metallicities, could provide additional constraints on star formation histories, and aid in the calibration of near-infrared surface brightness fluctuations for cosmological distance measurements.