ar X iv : a st ro - p h / 98 03 04 1 v 1 4 M ar 1 99 8 AN OLD CLUSTER IN NGC 6822

We present spectroscopy of two clusters in the dwarf irregular galaxy NGC 6822. From these we deduce an age for Cluster VII of 11 −3 Gyr and [Fe/H] = −1.95±0.15 dex. Cluster VII appears to be an analog of the metal-poor galactic globular clusters. Cluster VI is found to be much younger and more metal rich, with an age of approximately 2 Gyr. Its derived metallicity, [Fe/H] ≈ −1.0 dex, is comparable to that of the gas seen today in NGC 6822. The existence of a metal-poor old cluster in NGC 6822 rules out models for the chemical evolution of this galaxy with significant prompt initial enhancement. We find that a star formation rate which is constant with time and is within a factor of two of the present star formation rate can reproduce the two points on the age-metallicity relationship for NGC 6822 over the past 10 Gyr defined by these two clusters. Subject headings: Galaxies: individual (NGC 6822), Galaxies: dwarf, Galaxies: starclusters, Galaxies: abundances Accepted for publication in the Astronomical Journal. Based in large part on observations obtained at the W.M. Keck Observatory, which is operated jointly by the California Institute of Technology and the University of California Palomar Observatory, Mail Stop 105-24, California Institute of Technology