Stellar mass segregation as separating classifier between globular clusters and ultrafaint dwarf galaxies

We have determined the amount of stellar mass segregation in over 50 globular clusters and ultrafaint dwarf galaxy candidates based on deep HST- and ground-based photometry. We find that the amount of mass segregation in globular clusters is strongly correlated with their relaxation time and that all clusters with relaxation times of the order of their ages or longer have little to no mass segregation. For each cluster, the amount of mass segregation seen is fully compatible with the amount expected by dynamical evolution from initially unsegregated clusters, showing that globular clusters formed without primordial mass segregation among their low-mass stars. Ultrafaint dwarf galaxy candidates split into two groups, star clusters which follow the same trend between relaxation time and amount of mass segregation as globular clusters and dark-matter dominated dwarf galaxies that are unsegregated despite having relaxation times smaller than a Hubble time. Stellar abundance and velocity dispersion data, where available, confirm our classification. After classification of the ultrafaint dwarf galaxy candidates, we find that outer halo star clusters have average densities inside their half-light radii of 0.03 less than or similar to rho(h) 1 M-circle dot pc(-3), while dwarf galaxies have stellar densities of 0.001 less than or similar to rho(h) less than or similar to 0.03 M-circle dot pc(-3). The reason for this separation in density is most likely a combination of the initial conditions by which the systems formed and the requirement to withstand external tidal forces.

Automated summary

We have studied stellar mass segregation in over 50 globular clusters and ultrafaint dwarf galaxy candidates and found that the amount of segregation is strongly correlated with relaxation time. Globular clusters with relaxation times equal to or longer than their ages show little to no mass segregation, indicating that they formed without primordial mass segregation. Ultrafaint dwarf galaxy candidates can be divided into segregated star clusters and unsegregated dark-matter dominated dwarf galaxies, with the segregation being influenced by initial conditions and the ability to withstand external tidal forces.