The Brightest AGB Stars in the Inner Bulge of M31

Abstract

JHK images with angular resolutions approaching the diffraction limit of the 3.6 meter Canada-France-Hawaii Telescope are used to investigate the bright AGB content of the M31 bulge. The AGB-tip in a field 2.6 arcmin from the galaxy center occurs at K = 15.6, which is significantly fainter than measured in previous ground-based studies that sampled similar projected distances from the center of M31. Within 2.6 arcmin of the center of M31 the number density of bright AGB stars scales with r-band surface brightness, and the K brightness of the AGB-tip does not vary measureably with radius. It is concluded that the infrared bright AGB stars (1) belong to the bulge, and not the disk, and (2) are well mixed throughout the inner bulge, suggesting that they formed at a time when the overall structural properties of the M31 bulge were imprinted. The bolometric luminosity functions of the M31 bulge and Baade's Window are in excellent agreement, while the brightest AGB stars in the M31 bulge, the Galactic bulge, and M32 have similar M_K. These data suggest that the brightest stars in M32 and the bulges of M31 and the Milky-Way belong to an old, metal-rich population; these stars are bright not because they have a young or intermediate age, but because they have a high metallicity.

21 pages of text; 11 postscript figures. To appear in the AJ

References in corpus (1)

• Near-Infrared Imaging of the Central Regions of Metal-Poor Inner Spheroid Globular Clusters

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