Ages and Metallicities of Extragalactic Globular Clusters from Spectral and Photometric Fits of Stellar Population Synthesis Models

Spectra of galaxies contain an enormous amount of information about therelative mixture of ages and metallicities of constituent stars. We present acomprehensive study designed to extract the maximum information from spectra ofdata quality typical in large galaxy surveys. We test fitting techniques usingthe Bruzual-Charlot 2003 high resolution simple stellar population synthesismodels to simultaneously estimate the ages and metallicities of 101 globularclusters in M31 and the Magellanic Clouds. The clusters cover a wide range ofages and metallicities, 4 Myr to 20 Gyr and -1.6 < [Fe/H] < +0.3, estimated byother methods in the literature. We compare results from model fits to both thespectra and photometry and find that fits to continuum-normalized (CN) spectraover the entire range available, typically 350-1000 nm for this sample,provides the best results. For clusters older than 1 Gyr we agree withliterature ages to 0.16 dex (35%) and [Fe/H] to 0.12 dex. For younger clusterswe agree with literature ages to 0.3 dex (63%), but cannot constrain themetallicity. It is particularly important to use the entire CN spectrum toavoid problems with model continua for young objects and to breakage-metallicity degeneracies of broadband photometry. Our required S/N = 15-30for 20% age uncertainties and 30-55 for 10% uncertainties over spectralresolutions of 5-25 Angstroms. This technique should work well for theage-metallicity parameter space expected for early-type galaxies at z~1,although individual galaxy spectral S/N may require the coaddition of a fewlike objects.Comment: 70 pages, 18 figures, ApJ, in pres