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Color-magnitude diagrams (CMDs) derived from Hubble Space Telescope (HST)Advanced Camera for Surveys F606W,F814W photometry of 6 globular clusters (GCs)are presented. The six GCs form two loose groupings in Galactocentric distance(Rgc): IC 4499, NGC 6426, and Ruprecht 106 at ~15-20 kpc and NGC 7006, Palomar15, and Pyxis at ~40 kpc. The CMDs allow the ages to be estimated from the mainsequence turnoff in every case. In addition, the age of Palomar 5 (Rgc ~ 18kpc) is estimated using archival HST Wide Field Planetary Camera 2 V,Iphotometry. The age analysis reveals the following: IC 4499, Ruprecht 106, andPyxis are 1-2 Gyr younger than inner halo GCs with similar metallicities; NGC7006 and Palomar 5 are marginally younger than their inner halo counterparts;NGC 6426 and Palomar 15, the two most metal-poor GCs in the sample, are coevalwith all the other metal-poor GCs within the uncertainties. Combined with ourprevious efforts, the current sample provides strong evidence that the GalacticGC age-metallicity relation consists of two distinct branches. One suggests arapid chemical enrichment in the inner Galaxy while the other suggestsprolonged GC formation in the outer halo. The latter is consistent with theouter halo GCs forming in dwarf galaxies and later being accreted by the MilkyWay.

Globular clusters in the outer Galactic halo: new HST/ACS imaging of 6 globular clusters and the Galactic globular cluster age-metallicity relation