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Session 11 - Elliptical Galaxies.
Display session, Monday, June 10
We present a V-I color magnitude diagram for the central region of M32 based on Hubble Space Telescope WFPC-2 images. The broad color-luminosity distribution of red giants clearly shows that the stellar population of M32 comprises stars with a wide range in metallicity. The blue side of the giant branch rises to M_I \approx -4.0, and can be fitted with isochrones having [Fe/H]\approx-1.5. There is a heavily populated and dominant red sequence, however, that tops out at M_I \approx -3.2, and extends beyond V-I=4. This sequence can be fitted with isochrones with -0.2<[Fe/H]<+0.1, for ages running from 15Gyr to 5Gyr. At fainter magnitudes, the giant branch is significantly wider (FWHM_V - I\sim 0.6 mag down to M_I\sim-1.0) than can be accounted for by photometric uncertainties. The distribution of metallicities in M32 is considerably more peaked than that of the one-zone ``simple model" of chemical evolution, and appears somewhat more peaked than that of the solar neighborhood, although this last conclusion is weakened if there is a strong few-Gyr-old subpopulation present. There is no evidence for an extended or even red horizontal branch, but we find a strong clump on the giant branch itself. The complete lack of horizontal branch stars argues that most of the metal-poor population in M32 is younger than 10Gyr. We do not see the very bright asymptotic giant branch stars found in previous ground-based work, and argue that the majority of them were artifacts of crowding.
Program listing for Monday