Properties of young star cluster systems : the age signature from near-infrared integrated colours

Abstract

Context. A recent JHKs study of several grand-design spiral galaxies, including NGC 2997, shows a bimodal distribution of their system of star clusters and star forming complexes in colour-magnitude and colour-colour diagrams. In a comparison with stellar population models including gas, the (J − H) vs. (H − Ks) diagram reveals that embedded clusters, still immersed in their parental clouds of gas and dust, generally have a redder (H − Ks) colour than older clusters, whose gas and dust have already been ejected. This bimodal behaviour is also evident in the colour-magnitude diagram Mk vs. (J − Ks), where the brightest clusters split into two sequences separating younger from older clusters. In addition, the reddening-free index Qd = (H − Ks) − 0.884 (J − H) has been shown to correlate with age for the young clusters and thus provided an effective way to differentiate the embedded clusters from the older ones. Aims. We aim to study the behaviour of these photometric indices for star cluster systems in the Local Group. In particular, we investigate the effectiveness of the Qd index in sorting out clusters of different ages at their early evolutionary stages. In addition, the whole set of homogeneous measurements will serve as a template for analyses of the populations belonging to distant galaxies that are unresolved clusters or complexes. Methods. Surface photometry was carried out for 2MASS images of populous clusters younger than ~100 Myr whose ages were available. The integrated magnitude and colours were measured to a limiting radius and combined to generate the photometric diagrams. Some clusters, particularly the embedded ones, were studied for the first time using this method. Results. The integrated magnitudes and colours extracted from the surface photometry of the most populous clusters/complexes in the Local Group show the expected bimodal distribution in the colour–colour and colour–magnitude diagrams. In particular, we confirm the index Qd as a powerful tool for distinguishing clusters younger than about 7Myr from older clusters.