Author(s): , , , , , , ,
Institution(s): 1. Columbia University, 2. ESA, 3. ESO, 4. Institute for Advanced Studies in Basic Science, 5. Universität Bonn, 6. Universität Heidelberg, 7. University of Queensland
A good fraction of the globular clusters in the Milky Way halo show large half-light radii and flat stellar mass functions, depleted in low-mass stars. This is contrary to expectations since the driver of low-mass star depletion, two-body relaxation, should be least efficient in these clusters. Using a comprehensive set of direct N-body simulations of globular clusters on eccentric orbits within a Milky-Way-like potential, we show how a cluster's half-mass radius (or concentration) and its mass function develop over time due to two-body relaxation and tidal shocks. The slope of the stellar mass function flattens proportionally to the amount of mass a cluster has lost, and the half-mass radius grows to a size proportional to the average strength of the tidal field. The main driver of these processes are black holes, neutron stars and white dwarfs segregating into the cluster center, and sending low-mass stars on wide radial orbits, where they can be preferentially stripped from the cluster. We conclude that the extended, depleted clusters observed in the Milky Way must have had small half-mass radii in the past, and that they expanded due to the weak tidal field they spend most of their lifetime in. Moreover, their mass functions must have been steeper in the past but flattened significantly as a cause of tidal mass loss and mass segregation. As a consequence of this process, we predict outer-halo clusters to be mass segregated and have a strong radial anisotropy at large cluster radii.