Hubble Space Telescope observations of the Hubble clusters in NGC 6822:: Ages and structure

We present Hubble Space Telescope observations of four cluster candidates in NGC 6822 originally identified by Hubble. One of them, Hubble IV, is an H II region while Hubble VI, VII, and VIII are confirmed as rich star clusters. Based upon cluster color-magnitude diagrams from which the field star contamination has been subtracted, we derive ages of 70 +/- 10 Myr for Hubble VI and 1.5 +/- 0.2 Gyr for Hubble VIII. While the age is more uncertain in the case of Hubble VII, the available evidence argues that this cluster is probably similar to the old, metal-poor globular clusters in the Milky Way. Comparisons of the cluster ages and integrated magnitudes with models of cluster evolution imply that the two young clusters are an order of magnitude less massive than the older cluster Hubble VII. The radial profiles of all three clusters are reasonably well fit by empirical King models. The core radius and central surface brightness of Hubble VII are consistent with observations of Galactic globular clusters. The half-light radii of these clusters are comparable to values for the inner Milky Way globulars and are on average smaller than clusters in the Magellanic Clouds. These clusters apparently are exceptions to the trend noted by van den Bergh that larger clusters tend to form in environments with lower stellar densities.