Estimating the ages of early-type galaxies

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 first testing model fits 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 < t age < 20 Gyr and -1.6 < [Fe/H] < +0.3, estimated by other methods in the literature. We compare results from fits to both spectra and photometry and find that fits to continuum-normalized spectra over the entire range available, typically 3500 A to 1 mum, 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 A-1 for Deltalambda = 10--25 A.; We apply these techniques to 45 early-type galaxies from the MU-NICS survey, with zmed=0.362, and to 22 local elliptical galaxies and find that the best procedure for galaxies is to combine fits to the continuum-normalized 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 fits 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 z f 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 refinement of the technique, multiple population fits will be completely analyzed to characterize the formation histories of early-type galaxies.