Galactic globular cluster relative ages

Based on a new large, homogeneous photometric database of 34 Galactic globu lar clusters (GGCs; plus Palomar 12), a set of distance- and reddening- ind ependent relative age indicators has been measured. The observed delta(V-I) (2.5) and Delta V-TO(HB) versus metallicity relations have been compared wi th the relations predicted by two recently updated libraries of isochrones. Using these models and two independent methods, we have found that self-co nsistent relative ages can be estimated for our GGC sample. In turn, this d emonstrates that the models are internally self-consistent. Based on the re lative age versus metallicity distribution, we conclude that (1) there is n o evidence of an age spread for clusters with [Fe/H] < -1.2, all the cluste rs of our sample in this range being old and coeval; (2) for the intermedia te-metallicity group (-1.2 I [Fe/H] < -0.9), there is a clear evidence of a ge dispersion, with clusters up to similar to 25% younger than the older me mbers; and (3) the clusters within the metal-rich group ([Fe/H] greater tha n or equal to -0.9) seem to be coeval within the uncertainties (except Pal 12) but younger (similar to 17%) than the bulk of the GGCs. The latter resu lt is totally model dependent. From the Galactocentric distribution of the GGC ages, we can divide the GGCs in two groups: the old, coeval clusters an d the young clusters. The second group can be divided into two subgroups: t he "really young clusters" and the "young, but model dependent," which are within the intermediate- and high-metallicity groups, respectively. From th is distribution, we can present a possible scenario for the Milky Way's for mation: The globular cluster formation process started at the same zero age throughout the halo, at least out to similar to 20 kpc from the Galactic c enter. According to the present stellar evolution models, the metal-rich cl usters are formed at a later time (similar to 17% lower age). Finally, sign ificantly younger halo GGCs are found at any R-GC > 8 kpc. For these, a pos sible scenario associated with mergers of dwarf galaxies to the Milky Way i s suggested.