Using H-alpha Morphology and Surface Brightness Fluctuations to Age-Date Star Clusters in M83

Bradley C. Whitmore, Rupali Chandar, Hwihyun Kim, Catherine Kaleida, Max Mutchler, Daniela Calzetti, Abhijit Saha, Robert O'Connell, Bruce Balick, Howard E. Bond, Marcella Carollo, Michael J. Disney, Michael A. Dopita, Jay A. Frogel, Donald N. B. Hall, Jon A. Holtzman, Randy A. Kimble, Patrick J. McCarthy, Francesco Paresce, Joseph I. Silk, John T. Trauger, Alistair R. Walker, Rogier A. Windhorst, Erick T. Young

Published 2011-03-21Version 1

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 Halpha emission to estimate the ages of clusters younger than tau ~10 Myr. It is based on the simple premise that the gas in very young (tau < few Myr) clusters is largely coincident with the cluster stars, is in a small, ring-like structure surrounding the stars in slightly older clusters (e.g., tau ~5 Myr), and is in a larger ring-like bubble for still older clusters (i.e., ~5-10 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. 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 UBVI Halpha 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 tau 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" HII regions in M83, with central stars having ages ~4 Myr.