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. Ou r color-magnitude diagram in (V, V-I) reaches V-lim similar or equal t o 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'' clu ster 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 similar to 1 Gyr precision of measurement. The addition of the remote-halo object N GC 2419 to this list leads us to conclude that the earliest star (or g lobular cluster) formation began at essentially the same rime everywhe re in the Galactic halo throughout a region now almost 200 kpc in diam eter. Thus for the metal-poorest clusters in the halo there is no dete ctable age gradient with Galactocentric distance. To estimate the abso lute age of NGC 2419 and M92, we fit newly computed isochrones transfo rmed through model-atmosphere calculations to the (M-V, V-I) plane, wi th assumed distance scales that represent the range currently debated in the literature. Unconstrained isochrone fits give M-V(RR) similar o r equal to 0.55 +/- 0.06 for both clusters, and a resulting age of 14 to 15 Gyr. Incorporating the full effects of helium diffusion would fu rther reduce this estimate by similar to 1 Gyr, The first reports of H ipparcos parallax measurements for the lowest-metallicity subdwarfs su ggest that the distance scale could be as bright as M-V(RR) = 0.15 for [Fe/H] similar or equal to -2, which would require the cluster ages t o be less than 10 Gyr; however, the isochrone fits for a distance scal e this extreme leave several serious problems which have no obvious so lution in the context of current stellar models. (C) 1997 American Ast ronomical Society.