Age and metallicity gradients in the Galactic Bulge: A differential study using HST/WFPC2

The Galactic Bulge has long been assumed to be a largely old stellarpopulation. However, some recent studies based on observations with the HSTWF/PC-1 and WFPC2 of stars in the Galactic Bulge have concluded that the oldpopulation may not make up more then 300f the total. Other studies usingHST/WFPC2 differential studies of `Bulge' globular clusters and field starshave found the bulge to be comparable in age to the Galactic Halo. Acomplication in all these studies is the presence of a substantial populationof stars which mimic a young bulge population, but which may be, and are oftenassumed to be, foreground disk stars whose reddening and distance distributionshappen to mimic a young bulge turnoff. We show, using number counts inHST/WFPC2 colour-magnitude diagrams of both field stars in the Bulge and of two`bulge' and one `disk' globular cluster (NGC6528, NGC6553, and NGC5927) thatthe stars interpreted as young in fact are foreground disk stars. Thus, weconfirm that the bulk of the bulge field stars in Baade's Window are old. Theexistence of a young metal-rich population cannot, however, be ruled out fromour data. We also test for age and metallicity gradients using the CMD of ametal-rich globular cluster as an empirical isochrone and derive a metallicitydifference of