The Globular Cluster Relative Ages and the Milky Way Formation Time Scale

摘要

The ACS Survey of Galactic Globular Clusters is a Hubble Space Telescope (HST)Treasury program designed to provide a new large, deep and homogeneous photometric database.Based on observations from this program, we have measured precise relative ages for a sampleof 64 Galactic globular clusters by comparing the relative position of the clusters' main sequenceturn offs, using main-sequence fitting to cross-compare clusters within the sample. This methodprovides relative ages to a formal precision of 2-7%. We demonstrate that the calculated relativeages are independent of the choice of theoretical model. We find that the Galactic globular clustersample can be divided into two groups — a population of old clusters with an intrinsic age dispersionof ～3% and no age-metallicity relation, and a group of younger clusters with an age-metallicityrelation similar to that of the globular clusters associated with the Sagittarius dwarf galaxy. These results are consistent with the Milky Way halo having formed in two phases. The first phasewould be compatible with a rapid (<0.8 Gyr) assembling process of the halo, in which the clustersin the old group were formed. The second phase lasted much longer in time and resulted in a groupof globular clusters with a clear age-metallicity relation. It is very tempting to argue that the originof this second group of clusters is related to the accretion of Milky Way satellite galaxies, but theorigin of the age-metallicity relation remains unclear.