Some Global Characteristics of the Galactic Globular Cluster System

Abstract

The relations between the luminosities MV, the metallicities [Fe/H], the Galactocentric radii R, and the central concentration indices c of Galactic globular clusters are discussed. It is found that the most luminous clusters rarely have collapsed cores. The reason for this might be that the core collapse timescales for such populous clusters are greater than the age of the Galaxy. Among those clusters for which the structure has not been modified by core collapse, there is a correlation between central concentration and integrated luminosity, in the sense that the most luminous clusters have the strongest central concentration. The outermost region of the Galaxy with R > 10 kpc was apparently not able to form metal-rich ([Fe/H] > -1.0) globular clusters, whereas such clusters (of which Ter 7 is the prototype) were able to form in some nearby dwarf spheroidal galaxies. It is not yet clear how the popular hypothesis that globular clusters were initially formed with a single power-law mass spectrum can be reconciled with the observation that both (1) Galactic globular clusters with R > 80 kpc and (2) the globulars associated with the Sagittarius dwarf appear to have bimodal luminosity functions. Presently available data hint at, but are not yet able to establish, the existence of a radial metallicity gradient for metal-poor globular clusters with [Fe/H] < -1.0.