Preprint typeset using LATEX style emulateapj v. 08/22/09 THE STAR FORMATION HISTORY OF M32 1

We use deep HST ACS/HRC 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 mass-weighted mean age and metallicity of M32 at F1 are 〈Age 〉 = 6.8±1.5Gyr and 〈[M/H] 〉 = −0.01±0.08dex. The SFH additionally indicates the presence of young (< 2 Gyr old), metal-poor ([M/H] ∼ −0.7) stars, suggesting that blue straggler stars (BSS) contribute ∼ 2 % of the mass at F1; the remaining ∼ 3 % of the mass 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 ( ∼ 2.7re). This contradicts spectroscopic studies that show a steep age gradient from M32’s center to 1re. 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 a 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 ± 0.10dex