2011ApJ...729...78W

We use new WFC3 observations of the nearby grand-design spiral galaxy M83 to develop two independent methods for estimating the ages of young star clusters. The first method uses the physical extent and morphology of Hα emission to estimate the ages of clusters younger than τ ~ 10 Myr. It is based on the simple premise that the gas in very young (τ< a few Myr) clusters is largely coincident with the cluster stars, is in a small, ring-like structure surrounding the stars in slightly older clusters since massive star winds and supernovae have had time to push out the natal gas (e.g., τ ~ 5 Myr), and is in a larger ring-like bubble for still older clusters (i.e., ~5-10 Myr). If no Hα is associated with a cluster it is generally older than ~10 Myr. The second method is based on an observed relation between pixel-to-pixel flux variations within clusters and their ages. This method relies on the fact that the brightest individual stars in a cluster are most prominent at ages around 10 Myr, and fall below the detection limit (i.e., M_V< -3.5) for ages older than about 100 Myr. Older clusters therefore have a smoother appearance and smaller pixel-to-pixel variations. The youngest clusters also have lower flux variations, hence the relationship is double valued. This degeneracy in age can be broken using other age indicators such as Hα morphology. These two methods are the basis for a new morphological classification system which can be used to estimate the ages of star clusters based on their appearance. We compare previous age estimates of clusters in M83 determined from fitting UBVIHα measurements using predictions from stellar evolutionary models with our new morphological categories and find good agreement, at the ~95% level. The scatter within categories is ~0.1 dex in log τ for young clusters (<10 Myr) and ~0.5 dex for older (>10 Myr) clusters. A by-product of this study is the identification of 22 "single-star" H II regions in M83, with central stars having ages ~4 Myr.