Tk. Wyder et al., Hubble Space Telescope observations of the Hubble clusters in NGC 6822: Ages and structure, PUB AST S P, 112(775), 2000, pp. 1162-1176
Citations number
35
Categorie Soggetti
Space Sciences
Journal title
PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC
We present Hubble Space Telescope observations of four cluster candidates i
n 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 cluste
rs. Based upon cluster color-magnitude diagrams from which the field star c
ontamination has been subtracted, we derive ages of 70 +/- 10 Myr for Hubbl
e VI and 1.5 +/- 0.2 Gyr for Hubble VIII. While the age is more uncertain i
n the case of Hubble VII, the available evidence argues that this cluster i
s probably similar to the old, metal-poor globular clusters in the Milky Wa
y. Comparisons of the cluster ages and integrated magnitudes with models of
cluster evolution imply that the two young clusters are an order of magnit
ude less massive than the older cluster Hubble VII. The radial profiles of
all three clusters are reasonably well fit by empirical King models. The co
re radius and central surface brightness of Hubble VII are consistent with
observations of Galactic globular clusters. The half-light radii of these c
lusters are comparable to values for the inner Milky Way globulars and are
on average smaller than clusters in the Magellanic Clouds. These clusters a
pparently are exceptions to the trend noted by van den Bergh that larger cl
usters tend to form in environments with lower stellar densities.