Ages have been derived for 55 globular clusters ( GCs ) for which Hubble Space Telescope ACS photometry is publicly available .
For most of them , the assumed distances are based on fits of theoretical zero-age horizontal branch ( ZAHB ) loci to the lower bound of the observed distributions of HB stars , assuming reddenings from empirical dust maps and metallicities from the latest spectroscopic analyses .
The age of the isochrone that provides the best fit to the stars in the vicinity of the turnoff ( TO ) is taken to be the best estimate of the cluster age .
The morphology of isochrones between the TO and the beginning part of the subgiant branch ( SGB ) is shown to be nearly independent of age and chemical abundances .
For well-defined CMDs , the error bar arising just from the “ fitting ” of ZAHBs and isochrones is \approx \pm 0.25 Gyr , while that associated with distance and chemical abundance uncertainties is \sim \pm 1.5 –2 Gyr .
The oldest GCs in our sample are predicted to have ages of \approx 13.0 Gyr ( subject to the aforementioned uncertainties ) .
However , the main focus of this investigation is on relative GC ages .
In conflict with recent findings based on the relative main-sequence fitting ( rMSF ) method , which have been studied in some detail and reconciled with our results , ages are found to vary from mean values of \approx 12.5 Gyr at [ Fe/H ] \mathrel { \hbox { \raise 2.58 pt \hbox { $ < $ } \kern - 7.74 pt \lower 2.15 pt \hbox { $ \sim$ } } } % -1.7 to \approx 11 Gyr at [ Fe/H ] \mathrel { \hbox { \raise 2.58 pt \hbox { $ > $ } \kern - 7.31 pt \lower 2.15 pt \hbox { $ \sim$ } } } -1 .
At intermediate metallicities , the age-metallicity relation ( AMR ) appears to be bifurcated : one branch apparently contains clusters with disk-like kinematics , whereas the other branch , which is displaced to lower [ Fe/H ] values by \approx 0.6 dex at a fixed age , is populated by clusters with halo-type orbits .
The dispersion in age about each component of the AMR is \sim \pm 0.5 Gyr .
There is no apparent dependence of age on Galactocentric distance ( R _ { G } ) nor is there a clear correlation of HB type with age .
As previously discovered in the case of M 3 and M 13 , subtle variations have been found in the slope of the SGB in the color-magnitude diagrams ( CMDs ) of other metal-poor ( [ Fe/H ] \mathrel { \hbox { \raise 2.58 pt \hbox { $ < $ } \kern - 7.74 pt \lower 2.15 pt \hbox { $ \sim$ } } } % -1.5 ) GCs .
They have been tentatively attributed to cluster-to-cluster differences in the abundance of helium .
Curiously , GCs that have relatively steep “ M 13-like ” SGBs tend to be massive systems , located at small R _ { G } , that show the strongest evidence of in situ formation of multiple stellar populations .
The clusters in the other group are typically low-mass systems ( with 2–3 exceptions , including M 3 ) that , at the present time , should not be able to retain the matter lost by mass-losing stars due either to the development of GC winds or to ram-pressure stripping by the halo interstellar medium .
The apparent separation of the two groups in terms of their present-day gas retention properties is difficult to understand if all GCs were initially \sim 20 times their current masses .
The lowest mass systems , in particular , may have never been massive enough to retain enough gas to produce a significant population of second-generation stars .
In this case , the observed light element abundance variations , which are characteristic of all GCs , were presumably present in the gas out of which the observed cluster stars formed .