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

Abstract

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