The Formation of Galactic Spheroids

Abstract

The spheroidal components of galaxies, including globular clusters, are usually assumed to form during a period of rapid collapse and high luminosity. A few years ago, I developed in collaboration with Martin Rees, a theory for the origin of globular clusters (Fall and Rees 1985. 1988). We showed that realistic density or velocity perturbations in a protogalaxy would be amplified during the collapse. The result is a two-phase medium with some gas at the virial temperature, a few × 10⁶K, and some gas at the temperture that hydrogen recombines, about 10⁴K. The density of the hot gas can be derived from fairly general arguments, and since the two phases must be in rough pressure balance, the density and Jeans mass of the cold gas can be calculated with some confidence. We found that clouds with masses of order 10⁶ M. would collapse gravitationally, and should therefore be identified as the progenitors of globular clusters. The condition for this mass scale to be “imprinted” is that the temperatures of the clouds remain near 10⁴K for at least one free-fall time. Once the protogalactic gas has been enriched in heavy elements from the first generation of stars in globular clusters, cooling becomes more efficient and much smaller clouds can collapse, thereby promoting the formation of later generations of field stars. We therefore expect that on average globular clusters should have lower abundances of heavy elements and more extended space distributions than the field stars in the spheroidal components of galaxies. As the result of various selection biases, these predictions are not easy to test for the Milky Way but they are consistent with a growing body of data for other galaxies (Forte, Strom and Strom 1981, Harris 1986, 1987, Mould 1987, Mould, Oke and Nemec 1987, Elson and Walterbos 1987).