Infrared Surface Brightness Fluctuations of Magellanic Star Clusters

Abstract

We present surface brightness fluctuations (SBFs) in the near-IR for 191 Magellanic star clusters available in the Second Incremental and All Sky Data releases of the Two Micron All Sky Survey (2MASS) and compare them with SBFs of Fornax Cluster galaxies and with predictions from stellar population models as well. We also construct color-magnitude diagrams (CMDs) for these clusters using the 2MASS Point Source Catalog (PSC). Our goals are twofold. The first is to provide an empirical calibration of near-IR SBFs, given that existing stellar population synthesis models are particularly discrepant in the near-IR. Second, whereas most previous SBF studies have focused on old, metal-rich populations, this is the first application to a system with such a wide range of ages (~106 to more than 1010 yr, i.e., 4 orders of magnitude), at the same time that the clusters have a very narrow range of metallicities (Z ~ 0.0006-0.01, i.e., 1 order of magnitude only). Since stellar population synthesis models predict a more complex sensitivity of SBFs to metallicity and age in the near-IR than in the optical, this analysis offers a unique way of disentangling the effects of age and metallicity. We find a satisfactory agreement between models and data. We also confirm that near-IR fluctuations and fluctuation colors are mostly driven by age in the Magellanic cluster populations and that in this respect they constitute a sequence in which the Fornax Cluster galaxies fit adequately. Fluctuations are powered by red supergiants with high-mass precursors in young populations and by intermediate-mass stars populating the asymptotic giant branch in intermediate-age populations. For old populations, the trend with age of both fluctuation magnitudes and colors can be explained straightforwardly by evolution in the structure and morphology of the red giant branch. Moreover, fluctuation colors display a tendency to redden with age that can be fitted by a straight line. For the star clusters only, ( - s) = (0.21 ± 0.03) log(age) - (1.29 ± 0.22); once galaxies are included, ( - s) = (0.20 ± 0.02) log(age) - (1.25 ± 0.16). Finally, we use for the first time a Poissonian approach to establish the error bars of fluctuation measurements, instead of the customary Monte Carlo simulations.