Abstract

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 similar to 40% +/- 17% of M32' s mass, while similar to 55% +/- 21% of M32' s mass comes from stars older than 5 Gyr. The mass-weighted mean age and metallicity of M32 at F1 are Age > = 6.8 +/- 1.5Gyr and [M/H]> = -0.01 +/- 0.08 dex. The SFH additionally indicates the presence of young ( 2 Gyr old), metal-poor ([M/H] similar to -0.7) stars, suggesting that blue straggler stars contribute similar to 2% of themass at F1; the remaining similar to 3% of themass is in young metal-rich stars. Line-strength indices computed from the SFH imply a light-weighted mean age and metallicity of 4.9 Gyr and [M/H] = -0.12 dex, and single stellar population-equivalent parameters of 2.9 +/- 0.2 Gyr and [M/H] = 0.02 +/- 0.01 dex at F1 (similar to 2.7 r(e)). This contradicts spectroscopic studies that show a steep age gradient from M32' s center to 1 re. The inferred SFH of the M31 background field F2 reveals that the majority of its stars are old, with similar to 95% of its mass already acquired 5-14 Gyr ago. It is composed of two dominant populations; similar to 30% +/- 7.5% of its mass is in a 5-8 Gyr old population, and similar to 65% +/- 9% of the mass is in an 8-14 Gyr old population. The mass-weighted mean age and metallicity of F2 are Age > = 9.2 +/- 1.2Gyr and [M/H]> = -0.10 +/- 0.10 dex, respectively. 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 (similar to 1 disk scale length) to be similar to 5-9 Gyr, our results agree with an inside-out disk formation scenario for M31' s disk.