Abstract

In cold dark matter cosmological models, the first stars to form are believed to do so within small protogalaxies. We wish to understand how the evolution of these early protogalaxies changes once the gas forming them has been enriched with small quantities of heavy elements, which are produced and dispersed into the intergalactic medium by the first supernovae. We use high-resolution hydrodynamic simulations that incorporate the effects of the appropriate chemical and thermal processes. Our initial conditions represent protogalaxies forming within a fossil HII region, a previously ionized region that has not yet had time to cool and recombine. We study the influence of low levels of metal enrichment on the cooling and collapse of ionized gas in small protogalactic halos using three-dimensional, smoothed particle hydrodynamics simulations with particle splitting. In the centrally condensed potential that we study here, a wide variety of initial conditions for the gas yield a monolithic central collapse. Our models show no fragmentation during collapse to number densities as high as 10^{5 cm}â^ÈÒ3 for metallicities reaching as high as 10^âÈ^Ò1 Z_sun, far above the threshold suggested by previous work. We therefore argue that fragmentation at moderate density depends on the initial conditions for star formation more than on the metal abundances present.