Deep Photometry of Andromeda Reveals Striking Similarities in the Tidal Stream and Spheroid Populations1

We present a color-magnitude diagram (CMD) for a field in the giant tidal stream of the Andromeda galaxy (M31). These observations, taken with the Advanced Camera for Surveys on the Hubble Space Telescope, are 50% complete at V ≈ 30, reaching 1 mag below the oldest main-sequence turnoff. Striking similarities between the stream and a previous spheroid CMD imply they have very similar age and metallicity distributions, but present something of an enigma; we speculate on possible interpretations of this result, but note that none are without problems. Distinct multiple turnoffs, as might be expected from pulses of star formation caused by interaction with Andromeda, are not apparent in the stream CMD. Star formation in both fields lasted about 6 billion years, building up to relatively high metallicities and being largely complete 6 billion years ago. The close similarity of the spheroid and stream suggests that both may have derived from the same event; it would be worth exploring to what extent stars in these structures are the remnants of a disk galaxy that interacted with M31, or even were disrupted from the M31 disk itself by the interaction. Subject headings: galaxies: evolution – galaxies: stellar content – galaxies: halos – galaxies: individual (M31)