The Deepest Hubble Space Telescope Color-Magnitude Diagram of M32. Evidence for Intermediate-age Populations

Abstract

We present the deepest optical color-magnitude diagram (CMD) to date of the local elliptical galaxy M32. We have obtained F435W and F555W photometries based on Hubble Space Telescope (HST) Advanced Camera for Surveys/High-Resolution Channel images for a region 110" from the center of M32 (F1) and a background field (F2) about 320" away from M32 center. Due to the high resolution of our Nyquist-sampled images, the small photometric errors, and the depth of our data (the CMD of M32 goes as deep as F435W ~ 28.5 at 50% completeness level), we obtain the most detailed resolved photometric study of M32 yet. Deconvolution of HST images proves to be superior than other standard methods to derive stellar photometry on extremely crowded HST images, as its photometric errors are ~2× smaller than other methods tried. The location of the strong red clump in the CMD suggests a mean age between 8 and 10 Gyr for [Fe/H] = –0.2 dex in M32. We detect for the first time a red giant branch bump and an asymptotic giant branch (AGB) bump in M32 which, together with the red clump, allow us to constrain the age and metallicity of the dominant population in this region of M32. These features indicate that the mean age of M32's population at ~2' from its center is between 5 and 10 Gyr. We see evidence of an intermediate-age population in M32 mainly due to the presence of AGB stars rising to M _(F555W) ~ –2.0. Our detection of a blue component of stars (blue plume) may indicate for the first time the presence of a young stellar population, with ages of the order of 0.5 Gyr, in our M32 field. However, it is likely that the brighter stars of this blue plume belong to the disk of M31 rather than to M32. The fainter stars populating the blue plume indicate the presence of stars not younger than 1 Gyr and/or BSSs in M32. The CMD of M32 displays a wide color distribution of red giant branch stars indicating an intrinsic spread in metallicity with a peak at [Fe/H] ~ –0.2. There is not a noticeable presence of blue horizontal branch stars, suggesting that an ancient population with [Fe/H] < –1.3 does not significantly contribute to the light or mass of M32 in our observed fields. M32's dominant population of 8-10 Gyr implies a formation redshift of 1 ≲ z_f ≲ 2, precisely when observations of the specific star formation rates and models of "downsizing" imply galaxies of M32's mass ought to be forming their stars. Our CMD therefore provides a "ground truth" of downsizing scenarios at z = 0. Our background field data represent the deepest optical observations yet of the inner disk and bulge of M31. Its CMD exhibits a broad color spread of red giant stars indicative of its metallicity range with a peak at [Fe/H] ~ –0.4 dex, slightly more metal-poor than M32 in our fields. The observed blue plume consists of stars as young as 0.3 Gyr, in agreement with previous works on the disk of M31. The detection of bright AGB stars reveals the presence of intermediate-age population in M31, which is, however, less significant than that in M32 at our field's location.

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