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An Old Cluster in NGC 6822*

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© 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.
, , Citation Judith G. Cohen and John P. Blakeslee 1998 AJ 115 2356 DOI 10.1086/300389

1538-3881/115/6/2356

Abstract

     We present spectroscopy of two clusters in the dwarf irregular galaxy NGC 6822. From these we deduce an age for cluster VII of 11 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 that is constant with time and is within a factor of 2 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.

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Footnotes

  • 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.

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10.1086/300389