A robust method for the analysis of integrated spectra from globular clusters using Lick indices

ABSTRACT We define a method for analysis of the integrated spectra of extra-galactic globularclusters that provides more reliable measures of age, metallicity and α-element abun-dance ratio than have so far been achieved. The method involves the simultaneousfitting of up to 25 Lick indices in a χ 2 -fitting technique that maximises the use of theavailable data. Here we compare three sets of single stellar population (SSP) modelsof Lick indices to the high signal-to-noise, integrated spectra of 20 Galactic globu-lar clusters. The ages, [Fe/H] and α-element abundance ratios derived from the SSPmodels are compared to the results of resolved stellar population studies from theliterature. We find good consistency with the published values, with an agreement ofbetter than 0.1 dex in all three derived parameters. The technique allows identificationof abundance ratio anomalies, such as the known nitrogen over-abundance in Galac-tic globular clusters, and the presence of anomalous horizontal-branch morphologies.It also minimises the impact on the derived parameters of imperfect calibration tothe Lick system, and reduction errors in general. The method defined in this work istherefore robust with respect to many of the difficulties that plague the application ofSSP models in general, and is, consequently, well suited to the study of extra-galacticglobular cluster systems.Key words: stars: abundances – globular clusters: general – methods: analytical