NGC 2419, M92, and the age gradient in the galactic halo

The WFPC2 camera on HST has been used to obtain photometry of the low-metallicity (Fe/H = −2.14), outer-halo globular cluster NGC 2419. Our color-magnitude diagram in (V, V − I) reaches Vlim ≃ 27.8, clearly delineating the subgiant and turnoff region and about three magnitudes of the unevolved main sequence. A differential fit of the NGC 2419 CMD to that of the similarly metal-poor ‘standard ’ cluster M92 shows that they have virtually identical principal sequences and thus the same age to within 1 Gyr. Previously published studies of many other low-metallicity globular clusters throughout the Milky Way halo show that they possess this same age to within the ∼ 1 Gyr precision of measurement. The addition of the remote-halo object NGC 2419 to this list leads us to conclude that the earliest star (or globular cluster) formation began at essentially the same time everywhere in the Galactic halo throughout a region now almost 200 kpc in diameter. Thus for the metal-poorest clusters in the halo there is no detectable age gradient with Galactocentric distance. To estimate