Harvard-Smithsonian Center for Astrophysics
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(1) I determine the reddening and intrinsic colors of individual clusters, and find that the extinction laws in the Galaxy and M31 are not significantly different.
(2) I measure the distributions of M31 clusters' metallicities and metallicity-sensitive colors; both are bimodal with peaks at [Fe/H] ~ -1.4 and -0.6. The radial distribution and kinematics of the two M31 metallicity groups imply that they are analogs of the Galactic `halo' and `disk/bulge' cluster systems.
(3) I compare colors for M31 and Milky Way globular clusters to the predicted simple stellar population colors of three population synthesis models. The best-fitting models fit the cluster colors very well; offsets between model and data in the U and B passbands are likely due to problems with the spectral libraries used by the models. The best-fit models for the metal-rich clusters are younger than those for the metal-poor ones.
(4) I determine the globular cluster luminosity function (GCLF) parameters for several subsamples of the M31 globular cluster population. The inner third of the clusters have a brighter GCLF peak than the outer clusters, and the metal-poor clusters are fainter than the metal-rich clusters.
The results in (3) and (4) imply the globular cluster populations in M31 may have substantially different ages, which has important consequences for models of galaxy and cluster formation. I also use WFPC2 images from the Hubble Space Telescope Archive to model the selection effects in existing M31 cluster catalogs and measure structural parameters for globular clusters in M31. The M31 clusters have very similar structural parameters to the Galactic globulars, and are located on the same `fundamental plane'.