1999AJ....118.2306R

Based on a new large, homogeneous photometric database of 34 Galactic globular clusters (GGCs; plus Palomar 12), a set of distance- and reddening- independent relative age indicators has been measured. The observed δ(V-I)_2.5 and ΔV^HB_TO versus metallicity relations have been compared with the relations predicted by two recently updated libraries of isochrones. Using these models and two independent methods, we have found that self-consistent relative ages can be estimated for our GGC sample. In turn, this demonstrates that the models are internally self-consistent. Based on the relative age versus metallicity distribution, we conclude that (1) there is no evidence of an age spread for clusters with [Fe/H]←1.2, all the clusters of our sample in this range being old and coeval; (2) for the intermediate-metallicity group (-1.2≤[Fe/H]←0.9), there is a clear evidence of age dispersion, with clusters up to ∼25% younger than the older members; and (3) the clusters within the metal-rich group ([Fe/H]≥-0.9) seem to be coeval within the uncertainties (except Pal 12) but younger (∼17%) than the bulk of the GGCs. The latter result is totally model dependent. From the Galactocentric distribution of the GGC ages, we can divide the GGCs in two groups: the old, coeval clusters and the young clusters. The second group can be divided into two subgroups: the ``really young clusters'' and the ``young, but model dependent'', which are within the intermediate- and high-metallicity groups, respectively. From this distribution, we can present a possible scenario for the Milky Way's formation: The globular cluster formation process started at the same zero age throughout the halo, at least out to ∼20 kpc from the Galactic center. According to the present stellar evolution models, the metal-rich clusters are formed at a later time (∼17% lower age). Finally, significantly younger halo GGCs are found at any R_GC>8 kpc. For these, a possible scenario associated with mergers of dwarf galaxies to the Milky Way is suggested.