NEAR-COEVAL FORMATION OF THE GALACTIC BULGE AND HALO INFERRED FROM GLOBULAR-CLUSTER AGES

THE morphology of our Galaxy is characterized by a disk of stars movin g on circular orbits, surrounding a central spheroidal body of stars o n high-velocity, randomly oriented orbits. The spheroid is further dif ferentiated into an inner bulge and an outer halo; the bulge stars are rich in elements heavier than helium ('metals'), whereas the halo sta rs are metal-poor, suggesting that the latter formed very early in the history of the Galaxy. (They have experienced little chemical enrichm ent, by previous generations of stars.) It is not known, however, whet her the bulge is the inner extension of the halo, having formed as par t of the same process(1), or whether it formed much later, perhaps by a dynamical distortion of the inner regions of the disk(2,3). Here we report observations obtained with the Hubble Space Telescope of two me tal-rich globular clusters that form part of the bulge population. Wit hin the uncertainties, these bulge globular clusters appear to be coev al with halo clusters, which suggests that the formation of the bulge was part of the dynamical process that formed the halo, and that the b ulge gas underwent rapid chemical enrichment, in less than a few billi on years.