Globular clusters in the outer Galactic halo: new HST/ACS imaging of 6
  globular clusters and the Galactic globular cluster age-metallicity relation
  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, Palomar
15, and Pyxis at ~40 kpc. The CMDs allow the ages to be estimated from the main
sequence turnoff in every case. In addition, the age of Palomar 5 (Rgc ~ 18
kpc) is estimated using archival HST Wide Field Planetary Camera 2 V,I
photometry. The age analysis reveals the following: IC 4499, Ruprecht 106, and
Pyxis are 1-2 Gyr younger than inner halo GCs with similar metallicities; NGC
7006 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 coeval
with all the other metal-poor GCs within the uncertainties. Combined with our
previous efforts, the current sample provides strong evidence that the Galactic
GC age-metallicity relation consists of two distinct branches. One suggests a
rapid chemical enrichment in the inner Galaxy while the other suggests
prolonged GC formation in the outer halo. The latter is consistent with the
outer halo GCs forming in dwarf galaxies and later being accreted by the Milky
Way.
