The total mass M _ { \mathrm { GCS } } in the globular cluster ( GC ) system of a galaxy is empirically a near-constant fraction of the total mass M _ { h } \equiv M _ { bary } + M _ { dark } of the galaxy , across a range of 10 ^ { 5 } in galaxy mass .
This trend is radically unlike the strongly nonlinear behavior of total stellar mass M _ { \star } 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 \eta _ { M } = M _ { \mathrm { GCS } } / M _ { h } from normal galaxies , accounting for new revisions in the adopted mass-to-light ratio for GCs , now gives \eta _ { M } = 2.9 \times 10 ^ { -5 } as the mean absolute mass fraction .
We find that the same ratio appears valid for galaxy clusters and UDGs .
Estimates of \eta _ { 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 \eta _ { 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 \times 10 ^ { 12 } M _ { \odot } .
Physical explanations for the uniformity of \eta _ { 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 .