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 _ { eff } \approx 7000 – 12000 K , that is , when the Balmer Jump is strong .
This flat U - B vs T _ { eff } 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 vs 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 - 2.0 through 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 can not 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 .