The Globular Cluster M54 and the Star Formation History of the Sagittarius Dwarf Galaxy

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Abstract
We present a deep color-magnitude diagram in the VI passbands of the globular cluster M54, a member of the Sagittarius dwarf galaxy. The data extend below the cluster's main-sequence turnoff, allowing us to estimate the cluster's age. We find that M54 is 0.5-1.5 Gyr older than the Galactic globulars M68 and M5. In absolute terms, its age is comparable to the published age estimates of the other member clusters Arp 2 and Terzan 8 but is significantly greater than that of the member cluster Terzan 7. An age estimate of the Sagittarius field population relative to M54 suggests that M54 is 3 Gyr older than the field. We discuss briefly the star formation history of the Sagittarius dwarf galaxy.
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