The proto-galaxy, globular clusters, and quasars

Abstract

Metallicities and horizontal-branch population gradients are used to distinguish between two populations of Galactic halo globular clusters. The beta population of clusters appears to be almost coeval and occurs mainly at R less than R(solar). Clusters of the beta population were probably formed during a rapid Eggen, Lynden-Bell, and Sandage-type collapse of the inner proto-Galaxy. Clusters of the alpha population all occur at R greater than 8 kpc. Many of these objects are found to lie on plunging retrograde orbits. This suggests that clusters of the alpha population formed during an extended Searle-Zinn-type merger and collapse phase. The relative ages, metallicities, and locations of the alpha and beta populations indicate that the Galaxy formed inside out, with the dense proto-Galactic core forming stars and clusters before its lower density halo. In a massive galaxy the quasar phenomenon probably takes place after the bulk of its globular cluster system has formed. A quasar outburst may have guillotined cluster formation in the nearby S0 galaxy NGC 3115, thus preventing the formation of a thick-disk globular cluster population.