2018ApJ...863L..17N

Recently, van Dokkum et al. presented an important discovery of an ultra-diffuse galaxy, NGC1052-DF2, with a dark matter content significantly less than predicted from its stellar mass alone. The analysis relies on measured radial velocities of 10 globular clusters (GCs), of estimated individual masses of a few x10⁶ M_☉. This is about 1% of the inferred mass of NGC1052-DF2 of 2 x 10⁸ M_☉ within a half-light radius, R_e = 2.2 kpc. The large relative mass and the old age of these objects imply that they might be susceptible to orbital decay by dynamical friction. Using analytic estimates and N-body simulations of an isolated system matching the inferred mass profile of NGC1052-DF2, we show that the orbits of the most massive GCs should already have decayed on a timescale of a few Gyrs. These findings should help in constraining mass profile and formation scenarios of NGC1052-DF2.