Investigating the $M_{GCS}-M_h$ Relation in the Most Massive Galaxies

The relation between the total mass contained in the globular clusters of a galaxy and the mass of its dark matter halo has been found observationally to be nearly linear over five decades of mass. However, the high-mass end of this relation is not well determined from previous data and shows large scatter. We analyze the globular cluster systems (GCSs) of a homogeneous sample of 11 brightest cluster galaxies (BCGs) through DOLPHOT photometry of their deep Hubble Space Telescope (HST) images in the F814W filter. We standardize the definition of $M_{GCS}$, the total GCS mass, by using the GC total population within a limiting radius of $0.1 R_{virial}$, while the dark-matter halo mass $M_h$ is determined from the weak-lensing calibration of $M_h$ versus $M_{bary}$. When these 11 BCGs are added to the previously studied homogeneous catalogue of Virgo member galaxies, a total value for $\eta = M_{GCS}/M_h$ is found to be $(3.0\pm1.8_{internal})\times10^{-5}$, slightly higher than previous estimates but with much reduced uncertainty. Perhaps more importantly, the results suggest that the relation continues to have a near-linear shape at the highest galaxy masses, strongly reinforcing the conclusion that accreted GCs make a major contribution to the GC populations at high galaxy mass.