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 V_lim =~ 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 the absolute age of NGC 2419 and M92, we fit newly computed isochrones transformed through model-atmosphere calculations to the (M_V,V-I) plane, with assumed distance scales that represent the range currently debated in the literature. Unconstrained isochrone fits give M_V(RR) =~ 0.55 +/- 0.06 for both clusters, and a resulting age of 14 to 15 Gyr. Incorporating the full effects of helium diffusion would further reduce this estimate by ~ 1 Gyr. The first reports of Hipparcos parallax measurements for the lowest-metallicity subdwarfs suggest that the distance scale could be as bright as M_V(RR) = 0.15 for [Fe/H] =~ -2, which would require the cluster ages to be less than 10 Gyr; however, the isochrone fits for a distance scale this extreme leave several serious problems which have no obvious solution in the context of current stellar models.