A Comparison of Optical and Near-Infrared Colours of Magellanic Cloud Star Clusters with Predictions of Simple Stellar Population Models

TL;DR

This study compares optical and near-infrared colours of Magellanic Cloud star clusters with stellar population models, assessing their accuracy across different ages and accounting for stochastic fluctuations in low-mass clusters.

Contribution

It provides a comprehensive evaluation of SSP models against observed cluster colours and introduces composite colours to distinguish clusters by age and metallicity.

Findings

Models fit old clusters well but vary at younger ages.

Composite colours effectively differentiate age groups.

Stochastic fluctuations impact colour measurements in low-mass clusters.

Abstract

We present integrated JHK_s 2MASS photometry and a compilation of integrated-light optical photoelectric measurements for 84 star clusters in the Magellanic Clouds. These clusters range in age from ~200 Myr to >10 Gyr, and have [Fe/H] values from -2.2 to -0.1 dex. We find a spread in the intrinsic colours of clusters with similar ages and metallicities, at least some of which is due to stochastic fluctuations in the number of bright stars residing in low-mass clusters. We use 54 clusters with the most reliable age and metallicity estimates as test particles to evaluate the performance of four widely used SSP models in the optical/NIR colour-colour space. All models reproduce the reddening-corrected colours of the old (>10 Gyr) globular clusters quite well, but model performance varies at younger ages. In order to account for the effects of stochastic fluctuations in individual clusters, we provide composite B-V, B-J, V-J, V-Ks and J-Ks colours for Magellanic Cloud clusters in several different age intervals. The accumulated mass for most composite clusters are higher than that needed to keep luminosity variations due to stochastic fluctuations below the 10% level. The colours of the composite clusters are clearly distinct in optical-NIR colour-colour space for the following intervals of age: >10 Gyr, 2-9 Gyr, 1-2 Gyr, and 200 Myr-1 Gyr. This suggests that a combination of optical plus NIR colours can be used to differentiate clusters of different age and metallicity.