Seeing Red in M32: Constraints on the Stellar Content from Near- and Mid-Infrared Observations and Applications for Studies of More Distant Galaxies

Abstract

The properties of asymptotic giant branch (AGB) stars in the Local Group galaxy M32 are investigated using ground and space-based observations that span the 1 - 8um wavelength interval, with the goal of demonstrating the utility of infrared observations as probes of stellar content. Comparisons with isochrones indicate that the brightest resolved stars in M32 have ages of a few Gyr, and are younger on average than AGB stars with the same intrinsic brightness in the outer disk of M31. Accounting for stellar variability is shown to be essential for modelling AGB luminosity functions (LFs). Model LFs that assume the star-forming history measured by Monachesi et al. (2012, ApJ, 745, 97) and the variability properties of Galactic AGB stars match both the K and [5.8] LFs of M32. Models also suggest that the slope of the [5.8] LF between M_[5.8] = -8.5 and --10.0 is sensitive to the mix of stellar ages, and a sizeable fraction of the stars in M32 must have an age older than 7 Gyr in order to match the [5.8] LF. The structural properties of M32 in the infrared are also investigated. The effective radii that are computed from near-infrared and mid-infrared isophotes are similar to those measured at visible wavelengths, suggesting that the stellar content of M32 is well-mixed. However, isophotes at radii > 16 arcsec (> 60 parsecs) in the near and mid-infrared are flatter than those at visible wavelengths. The coefficient of the fourth-order cosine term in the fourier expansion of isophotes changes from `boxy' values at r < 16 arcsec to `disky' values at r > 48 arcsec in [3.6] and [4.5]. The mid-infrared colors near the center of M32 do not vary systematically with radius, providing evidence of a well-mixed stellar content in this part of the galaxy.

To appear in The Astrophysical Journal

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Cited by in corpus (8)

• An Infrared Census of DUST in Nearby Galaxies with Spitzer (DUSTiNGS). IV. Discovery of High-Redshift AGB Analogs
• Carbon Stars in the Satellites and Halo of M31
• The Star-Forming Histories of the Nucleus, Bulge, and Inner Disk of NGC 5102: Clues to the Evolution of a Nearby Lenticular Galaxy
• A Spitzer Space Telescope survey of extreme Asymptotic Giant Branch stars in M32
• Cluster Glimpses with Raven: AO Corrected Near and Mid-Infrared Images of Glimpse C01 and Glimpse C02
• The Lop-sided Spiral Galaxy NGC 247: Clues to a Possible Interaction with NGC 253
• Infrared variable stars in the compact elliptical galaxy M32
• Red Variable Stars in Three M81 Group Dwarf Galaxies