Globular clusters as probes of galaxy evolution : NGC 5128

Abstract

We present the results of our analysis of the photometric data of globular clusters in the elliptical galaxy NGC 5128 (Cen A). We show that the integrated colour U - B can be an effective metallicity indicator for simple stellar populations. This is because it is sensitive to metallicity via the opacity effect but relatively insensitive to the effective main-sequence turn-off temperature of the population (and thus to age) when T ≈ 7000-12 000 K, that is, when the Balmer Jump is strong. This flat U - B versus T relation is a result of the fact that the blueing effect of the optical continuum with increasing temperature is temporarily stopped by the Balmer Jump, which becomes greater with temperature in this range. In this study we use U - B and B - V as metallicity and age indicators, respectively. We first show that the use of the U - B versus B - V two-colour diagram roughly yields the metallicities and ages of the Milky Way globular clusters independently determined, and then apply the technique to the clusters in NGC 5128. There is a large range in U - B, which corresponds to [Fe/H] of from -2.0 to over +0.3. The large uncertainties from the data and the models currently prevent us from pinning down their ages and metallicities. Although a constant age for all these clusters cannot be ruled out, there is a hint of the metal-rich clusters being younger. Significance of these results and caveats of the analysis are discussed.