2012MNRAS.421.1678Z

The number of methods used to study the properties of galaxies is increasing, and it is very important to test these methods. Galactic globular clusters (GCs) provide an excellent medium for such tests, because they can be considered as simple stellar populations. We present ages and metallicities for 40 Galactic GCs as determined from three publicly available techniques, namely the colour, Lick-index and spectrum-fitting methods, based on Bruzual & Charlot evolutionary population synthesis (EPS) models. By comparisons with the ages obtained from colour–magnitude diagrams (CMDs) and metallicities obtained from the spectra of stars, we are able to estimate the ability of these methods to determine GC parameters, which is essential. We find that: (i) for the metallicity, our derived metallicities agree with those derived from the spectra of stars – the Lick-index method is suitable for studying metallicity for stellar population systems in the range -1.5 ≲ [ Fe/H ] ≲ - 0.7 and the spectrum-fitting method is suitable for studying metallicity for stellar population systems in the range -2.3 ≲ [ Fe/H ] ≲ - 1.5; (ii) for the age, these three methods have difficulties in age determination – our derived ages are smaller (by about 2.0 Gyr, on average) than the results of CMDs for all three methods. We use Vazdekis and Maraston models to analyse whether our results are dependent on EPS models, and find that the tendency of these two models is the same as that of Bruzual & Charlot models. Our results are independent of the EPS models. In addition, our test is based on old GCs and our conclusions may hold for old stellar population systems.

Besides the age–metallicity degeneracy, we examine the possible effects of other factors (horizontal branch morphology, blue straggler stars, binary interactions and α-enhancement) and give a quantitative analysis of the influences of these factors on age determinations (except for α-enhancement). For the colour and spectrum-fitting methods, the age can be underestimated by about 0.0–3.0, 0.0–2.0 and 0.0–3.0 Gyr owing to the influence of horizontal branch, blue straggler and binary stars, respectively. For the Lick-index method, the lower limit of maximal change of age is 6.0, 5.0 and 3.0 Gyr owing to the influence of horizontal branch, blue straggler and binary stars, respectively.