THE MAIN-SEQUENCE STARS OF THE SAGITTARIUS DWARF GALAXY

The Sagittarius dwarf galaxy (SDG) is visible in the background held o f the globular cluster M55. We present a deep VI color-magnitude diagr am (CMD) of M55, which shows a prominent sequence of stars some 3.5 ma g below the cluster main sequence. Through a comparison with a similar CMD for the globular cluster M4, we show that the M55 background fiel d is not the Galactic bulge or spheroid. The SDG main sequence is almo st as blue as that of M55 and thus, if it is metal rich, it must be yo unger than M55, a typical old Galactic globular cluster. The results f rom isochrone fitting indicate that the age of the SDG is 10-14 Gyr, s imilar to the ages inferred for the two associated globular clusters T er 7 and Arp 2.