The Formation History of Globular Clusters

Abstract

The properties of old globular cluster systems in galaxy halos are used to infer quantitative constraints on aspects of generic star (cluster) formation. First, the spatial distribution of globulars in three large galaxies, together with trends in total cluster population vs. galaxy luminosity for 97 early-type systems plus the halo of the Milky Way, imply that bound stellar clusters formed with a universal efficiency throughout early protogalaxies: by mass, always 0.26% of star-forming gas was converted into globulars rather than halo field stars. That this fraction is so robust in the face of extreme variations in local and global galaxy environment suggests that any parcel of gas needs primarily to exceed a relative density threshold in order to form a bound cluster of stars. Second, it is shown that a strict, empirical scaling of total binding energy with luminosity and Galactocentric position is a defining equation for a fundamental plane of Galactic globular clusters. The characteristics of this plane, which subsumes all other observable correlations between the structural parameters of globulars, provide a small but complete set of facts that must be explained by theories of cluster formation and evolution in the Milky Way. It is suggested that the E_b(L,r_{\rm gc}) relation specifically resulted from star formation efficiencies having been systematically higher inside more massive protoglobular gas clumps.