Estimating the ages of early-type galaxies
MetadataShow full item record
Spectra of galaxies contain much information about relative mixtures of ages and metallicities of constituent stars. We present a comprehensive study designed to extract the maximum information from spectra by rst testing model ts on a sample of globular clusters. We use the Bruzual-Charlot 2003 simple stellar population synthesis models (SSPs) to simultaneously estimate the ages and metallicities of 101 globular clusters in M31 and the Magellanic Clouds. The clusters cover a wide range of parameters, 4 Myr < tage < 20 Gyr and 1.6 < [Fe/H] < +0.3, estimated by other methods in the literature. We compare results from ts to both spectra and photometry and nd that ts to continuum-normalized spectra over the entire range available, typically 3500 A to 1 m, provide 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. The required S/N is 10-20 A1 for = 10-25 A. vii We apply these techniques to 45 early-type galaxies from the MUNICS survey, with zmed=0.362, and to 22 local elliptical galaxies and nd that the best procedure for galaxies is to combine ts to the continuumnormalized spectra and the BVRIJ photometry. Wide spectral coverage, rest frame 3700-9000 A, is required to break the age-metallicity degeneracy. Formation redshifts calculated from age estimates show a peak at zf=0.5-2 and, using our derived galaxy stellar masses, we see a transition at the same place from the formation of lower mass galaxies (M < 1010 M ) at z . 0.5 to the formation of higher mass galaxies (M > 1011 M ) at z & 1. Models containing 2 populations provide ts of equal quality to the SSPs and preliminary results indicate that young stellar populations contributing as little as 5% of the total luminosity in 40% of the galaxies can account for the peak at zf 1 with zf of the older populations nearly uniformly spread over z 0.5-10. Further tests are being conducted on a larger comparison sample of local galaxies with age and metallicity estimates derived from line indices. Upon re nement of the technique, multiple population ts will be completely analyzed to characterize the formation histories of early-type galaxies.