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

The WFPC2 camera on HST has been used to obtain deep main sequence photometryof the low-metallicity ([Fe/H]=-2.14), outer-halo globular cluster NGC 2419. Adifferential fit of the NGC 2419 CMD to that of the similarly metal-poor \standard cluster M92 shows that they have virtually identical principalsequences and thus the same age to well within 1 Gyr. Since otherlow-metallicity clusters throughout the Milky Way halo have this same age towithin the 1-Gyr precision of the differential age technique, we conclude thatthe earliest star (or globular cluster) formation began at essentially the sametime everywhere in the Galactic halo throughout a region now almost 200 kpc indiameter. Thus for the metal-poorest clusters in the halo there is nodetectable age gradient with Galactocentric distance. To estimate the absoluteage of NGC 2419 and M92, we fit newly computed isochrones transformed throughmodel-atmosphere calculations to the (M_V,V-I) plane, with assumed distancescales that represent the range currently debated in the literature.Unconstrained isochrone fits give M_V(RR) = 0.55 \pm 0.06 and a resulting ageof 14 to 15 Gyr. Incorporating the full effects of helium diffusion wouldfurther reduce this estimate by about 1 Gyr. A distance scale as bright asM_V(RR) = 0.15 for [Fe/H] = -2, as has recently been reported, would leaveseveral serious problems which have no obvious solution in the context ofcurrent stellar models.Comment: 32 pages, aastex, 9 postscript figures; accepted for publication in AJ, September 1997. Also available by e-mail from harris@physics.mcmaster.c