Galactic Dark Matter Halos and Globular Cluster Populations. III. Extension to Extreme Environments

The total mass in the globular cluster (GC) system of a galaxy is empirically a near-constant fraction of the total mass of the galaxy across a range of 105 in galaxy mass. This trend is radically unlike the strongly nonlinear behavior of total stellar mass M⋆ versus Mh. We discuss extensions of this trend to two more extreme situations: (a) entire clusters of galaxies and (b) the ultra-diffuse galaxies (UDGs) recently discovered in Coma and elsewhere. Our calibration of the ratio from normal galaxies, accounting for new revisions in the adopted mass-to-light ratio for GCs, now gives as the mean absolute mass fraction. We find that the same ratio appears valid for galaxy clusters and UDGs. Estimates of in the four clusters we examine tend to be slightly higher than for individual galaxies, but more data and better constraints on the mean GC mass in such systems are needed to determine if this difference is significant. We use the constancy of to estimate total masses for several individual cases; for example, the total mass of the Milky Way is calculated to be . Physical explanations for the uniformity of are still descriptive, but point to a picture in which massive dense star clusters in their formation stages were relatively immune to the feedback that more strongly influenced lower-density regions where most stars form.