The relative ages of Galactic globular clusters

We present a review of the present state of knowledge regarding the relative ages of Galactic globular clusters. First, we discuss the relevant galaxy formation models and describe the detailed predictions they make with respect to the formation timescale and chemical evolution of the globular clusters. Next, the techniques used to estimate globular cluster ages are described and evaluated with particular emphasis on the advantages and disadvantages of each method. With these techniques as a foundation, we present arguments in favor of the following assertions: (1) cluster age is the likeliest candidate to be the global second parameter, which, along with metal abundance, controls the morphology of the horizonal branch. (2) A total age range of as much as similar to 5 Gyr exists among the hulk of the Galactic globulars. (3) There is a significant relation between age and metallicity among the Galactic globular clusters if the slope of the M-v(RR)-[Fe/H] relation is less than similar to 0.23. These conclusions along with other supporting evidence favor a formation scenario in which the inner regions of the Galactic halo collapsed in a monotonic fashion over a short time period much less than 1 Gyr. In contrast, the outer regions of the halo fragmented and collapsed in a chaotic manner over several Gyrs.