2020AJ....159...56B

We confirm that the number of globular clusters (GCs), N_GC, is an excellent tracer of their host galaxy's halo virial mass, M_vir. The simple linear relation M_vir = 5 x 10⁹ M_☉ x N_GC fits the data perfectly from M_vir = 10¹⁰ M_☉ to M_vir = 2 x 10¹⁵ M_☉. This result is independent of galaxy morphology and extends statistically into the dwarf galaxy regime with M_vir = 10⁸-10¹⁰ M_☉, including the extreme ultra diffuse galaxy DF44. As this correlation does not depend on GC mass, it is ideally suited for high-precision determinations of M_vir. The linearity is most simply explained by cosmological merging of a high-redshift halo seed population that hosted on average one GC per 5 x 10⁸ M_☉ of dark matter. We show that hierarchical merging is also extremely powerful in restoring a linear correlation and erasing signatures of even a strong secular evolution of GC systems. The cosmological merging scenario also implies a strong decline of the scatter in N_GC with increasing virial mass δN_GC/N_GC∼M_vir^–1/2 in contrast with the observations that show a roughly constant scatter, independent of virial mass. This discrepancy can be explained if errors in determining virial masses from kinematical tracers and gravitational lensing are on the order of a factor of 2. GCs in dwarf satellite galaxies pose a serious problem for high-redshift GC formation scenarios; the dark halo masses of dwarf galaxies hosting GCs therefore might need to be an order of magnitude larger than currently estimated.