DARK MATTER HALOS IN GALAXIES AND GLOBULAR CLUSTER POPULATIONS

We combine a new, comprehensive database for globular cluster populations in all types of galaxies with a new calibration of galaxy halo masses based entirely on weak lensing. Correlating these two sets of data, we find that the mass ratio η ≡ MGCS/Mh (total mass in globular clusters, divided by halo mass) is essentially constant at 〈η〉 ∼ 4 × 10−5, strongly confirming earlier suggestions in the literature. Globular clusters are the only known stellar population that formed in essentially direct proportion to host galaxy halo mass. The intrinsic scatter in η appears to be at most 0.2 dex; we argue that some of this scatter is due to differing degrees of tidal stripping of the globular cluster systems between central and satellite galaxies. We suggest that this correlation can be understood if most globular clusters form at very early stages in galaxy evolution, largely avoiding the feedback processes that inhibited the bulk of field-star formation in their host galaxies. The actual mean value of η also suggests that about one-fourth of the initial gas mass present in protogalaxies collected into giant molecular clouds large enough to form massive, dense star clusters. Finally, our calibration of 〈η〉 indicates that the halo masses of the Milky Way and M31 are (1.2 ± 0.5) × 1012 M☉ and (3.9 ± 1.8) × 1012 M☉, respectively.