Galactic Globular Cluster Relative Ages

We present accurate relative ages for a sample of 55 Galactic globularclusters. The ages have been obtained by measuring the difference between thehorizontal branch and the turnoff in two, internally photometricallyhomogeneous databases. The mutual consistency of the two data sets has beenassessed by comparing the ages of 16 globular clusters in common between thetwo databases. We have also investigated the consistency of our relative agedetermination within the recent stellar model framework. All clusters with[Fe/H]<-1.7 are found to be old, and coeval, with the possible exception of twoobjects, which are marginally younger. The age dispersion for the metal poorclusters is 0.6 Gyr (rms), consistent with a null age dispersion. Intermediatemetallicity clusters (-1.7<[Fe/H]<-0.8) are on average 1.5 Gyr younger than themetal poor ones, with an age dispersion of 1.0 Gyr (rms), and a total age rangeof ~3 Gyr. About 15% of the intermediate metallicity clusters are coeval withthe oldest clusters. All the clusters with [Fe/H]>-0.8 are ~1 Gyr younger thanthe most metal poor ones, with a relatively small age dispersion, though themetal rich sample is still too small to allow firmer conclusions. There is nocorrelation of the cluster age with the Galactocentric distance. We brieflydiscuss the implication of these observational results for the formationhistory of the Galaxy.Comment: accepted for publication in the Astronomical Journa