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We present new Hubble Space Telescope photometry of the massive globularcluster M54 (NGC 6715) and the superposed core of the tidally disruptedSagittarius (Sgr) dSph galaxy as part of the ACS Survey of Galactic GlobularClusters. Our deep (F606W~26.5), high-precision photometry yields anunprecedentedly detailed color-magnitude diagram showing the extended bluehorizontal branch and multiple main sequences of the M54+Sgr system. Thedistance and reddening to M54 are revised usingboth isochrone and main-sequencefitting to (m-M)_0=17.27 and E(B-V)=0.15. Preliminary assessment finds theM54+Sgr field to be dominated by the old metal-poor populations of Sgr and theglobular cluster. Multiple turnoffs indicate the presence of at least twointermediate-aged star formation epochs with 4 and 6 Gyr ages and [Fe/H]=-0.4to -0.6. We also clearly show, for the first time, a prominent, 2.3 Gyr old Sgrpopulation of near-solar abundance. A trace population of even younger (0.1-0.8Gyr old), more metal-rich ([Fe/H]\sim0.6) stars is also indicated. The Sgrage-metallicity relation is consistent with a closed-box model and multiple(4-5) star formation bursts over the entire life of the satellite, includingthe time since Sgr began disrupting.

The ACS Survey of Galactic Globular Clusters: M54 and Young Populations in the Sagittarius Dwarf Spheroidal Galaxy