2017ApJ...836...67H

The total mass M_GCS in the globular cluster (GC) system of a galaxy is empirically a near-constant fraction of the total mass M_h≡M_bary+M_dark 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 M_h. 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 η_M=M_GCS/M_h from normal galaxies, accounting for new revisions in the adopted mass-to-light ratio for GCs, now gives η_M=2.9×10^–5 as the mean absolute mass fraction. We find that the same ratio appears valid for galaxy clusters and UDGs. Estimates of η_M 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 η_M to estimate total masses for several individual cases; for example, the total mass of the Milky Way is calculated to be M_h=1.1×10¹² M_☉. Physical explanations for the uniformity of η_M 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.