Properties of young star cluster systems: the age signature from
  near-infrared integrated colours
  A recent JHKs study of several grand-design spiral galaxies 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. We aim to study the behaviour of
these photometric indices for star cluster systems in the Local Group. We
investigate the effectiveness of the Qd index in sorting out clusters of
different ages at their early evolutionary stages. Surface photometry was
carried out for 2MASS images of populous clusters younger than ~100Myr whose
ages were available. Some clusters, particularly the embedded ones, were
studied for the first time using this method. The integrated magnitudes and
colours extracted from the surface photometry of the most populous
clusters/complexes in the Local Group shows 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. (abridged)
