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

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 turn-off, allowing us to estimate the cluster’s age. We find that M54 is 0.5–1.5 gigayears older than the Galactic globulars M68 and M5. In absolute terms, the age is comparable to the published age estimates of the other member clusters Arp 2 and Terzan 8, but is significantly older than the member cluster Terzan 7. An age estimate of the Sagittarius field population relative to M54 suggests that M54 is \gtrsim 3 Gyr older than the field. We discuss briefly the star formation history of the Sagittarius dwarf galaxy.