The Star Formation History of M32.

Fields observed with HST for M32 (F1) and M31 (F2) overplotted on a M32 image from ground.
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We use deep Hubble Space Telescope Advanced Camera for Surveys/High Resolution Channel observations of a field within M32 (F1) and an M31 background field (F2) to determine the star formation history (SFH) of M32 from its resolved stellar population. We find that 2-5 Gyr old stars contribute ~40% ± 17% of M32's mass, while ~55% ± 21% of M32's mass comes from stars older than 5 Gyr. The SFH additionally indicates the presence of young (<2 Gyr old), metal-poor ([M/H] ~ –0.7) stars, suggesting that blue straggler stars contribute ~2% of the mass at F1; the remaining ~3% of the mass is in young metal-rich stars. The inferred SFH of the M31 background field F2 reveals that the majority of its stars are old, with ~95% of its mass already acquired 5-14 Gyr ago. It is composed of two dominant populations; ~30% ± 7.5% of its mass is in a 5-8 Gyr old population, and ~65% ± 9% of the mass is in an 8-14 Gyr old population. Our results suggest that the inner disk and spheroid populations of M31 are indistinguishable from those of the outer disk and spheroid. Assuming the mean age of M31's disk at F2 (~1 disk scale length) to be ~5-9 Gyr, our results agree with an inside-out disk formation scenario for M31's disk.

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