The globular cluster systems of galaxies are well-known to extend to large galactocentric radii. Here we quantify the size of GC systems using the half number radius of 22 GC systems around early-type galaxies from the literature. We compare GC system sizes to the sizes and masses of their host galaxies. We find that GC systems typically extend to 4× that of the host galaxy size, however this factor varies with galaxy stellar mass from about 3× for M∗ galaxies to 5× for the most massive galaxies in the universe. The size of a GC system scales approximately linearly with the virial radius (R200) and with the halo mass (M200) to the 1/3 power. The GC system of the Milky Way follows the same relations as for early-type galaxies. For Ultra Diffuse Galaxies their GC system size scales with halo mass and virial radius as for more massive, larger galaxies. UDGs indicate that the linear scaling of GC system size with stellar mass for massive galaxies flattens out for low stellar mass galaxies. Our scalings are different to those reported recently by Hudson & Robison (2017).
Duncan A. Forbes