Galactic Globular Cluster Relative Ages

Based on a new large, homogeneous photometric database of 35 Galacticglobular clusters (GGCs), a set of distance and reddening independent relativeage indicators has been measured. The observed D(V-I)_2.5 and D(V)(HB-TO) vs.metallicity relations have been compared with the relations predicted by tworecent updated libraries of isochrones. Using these models and two independentmethods, we have found that self-consistent relative ages can be estimated forour GGC sample. Based on the relative age vs. metallicity distribution, weconclude that: (a) 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;(b) for the intermediate metallicity group (-1.2<=[Fe/H]<-0.9) there is a clearevidence of age dispersion, with clusters up to ~25% younger than the oldermembers; and (c) the clusters within the metal rich group ([Fe/H]>=-0.9) seemto be coeval within the uncertainties (except Pal12), but younger (~17%) thanthe bulk of the Galactic globulars. The latter result is totally modeldependent. From the distribution of the GGC ages with the Galactocentricdistance, we can present a possible scenario for the Milky Way formation: TheGC formation process started at the same zero age throughout the halo, at leastout to ~20 kpc from the Galactic center. According to the present stellarevolution models, the metal-rich globulars are formed at a later time (~ 17%lower age). And finally, significantly younger halo GGCs are found at anyR(GC)>8 kpc. For these, a possible scenario associated with mergers of dwarfgalaxies to the Milky Way is suggested.Comment: 47 pages, 9 figures. To be published in the Astronomical Journal, November issu