The Brightest Stars in M32: Comparing Predictions from Spectra with the Resolved Stellar Content

Broadband and narrowband images covering the 1-4 μm wavelength interval are used to investigate the properties of the brightest asymptotic giant branch (AGB) stars in the Local Group galaxy M32. Data obtained with the NIRI imager on the Gemini North telescope indicate that the brightest AGB stars near the center of M32 have peak M_L^' brightnesses and K-L^' colors that are similar to those of luminous AGB stars in the Galactic disk. Data obtained with the CFHTIR imager on the Canada-France-Hawaii Telescope indicate that the density of bright AGB stars per unit visible and near-infrared surface brightness is constant out to projected major-axis distances of 1 kpc, suggesting that the AGB stars and their progenitors are smoothly mixed throughout the main body of the galaxy. In addition, the J-K color distribution of bright AGB stars throughout much of the galaxy is consistent with that of a single population of AGB stars, the majority of which are long-period variables that have a common metallicity and age. Thus, these data do not support spectroscopic studies that find an age gradient in M32. The AGB contributes 70%^+30%_-20% of the integrated light in the region surveyed. This is consistent with previous estimates made from the integrated near-infrared spectrum and is suggestive of an age of ~2 Gyr. The stellar content of M32 is compared with that of the M31 bulge at a projected minor-axis distance of 1.4 kpc. While the peak K-band brightnesses of AGB stars in the two systems agree to within a few tenths of a magnitude, M32 contains more bright AGB stars per unit integrated brightness than the outer bulge of M31. This suggests a difference in mean age, and it is concluded that the star-forming histories of M32 and the bulge of M31 have differed over a significant fraction of their lifetimes, which is consistent with spectroscopic studies of these systems. The well-mixed AGB content of M32 is consistent with the galaxy having been tidally stirred, presumably by interactions with M31.

Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Particle Physics and Astronomy Research Council (United Kingdom), the National Research Council of Canada (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil), and CONICET (Argentina).

This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.