The Main-Sequence Stars of the Sagittarius Dwarf Galaxy

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Abstract
The Sagittarius dwarf galaxy (SDG) is visible in the background field of the globular cluster M55. We present a deep VI color-magnitude diagram (CMD) of M55, which shows a prominent sequence of stars some 3.5 mag below the cluster main sequence. Through a comparison with a similar CMD for the globular cluster M4, we show that the M55 background field is not the Galactic bulge or spheroid. The SDG main sequence is almost as blue as that of M55 and thus, if it is metal rich, it must be younger than M55, a typical old Galactic globular cluster. The results from isochrone fitting indicate that the age of the SDG is 10-14 Gyr, similar to the ages inferred for the two associated globular clusters Ter 7 and Arp 2.