Globular cluster populations and the kinematical fingerprints of minor mergers

Abstract

We use Monte Carlo lambda-cold dark matter assembly histories, minor-merger N-body simulations, and empirical relations between halo mass and the globular cluster (GC) abundance to study the kinematical properties of halo GCs in massive galaxies, with Mvir(z = 0) = 1013.5 M⊙. While the accreted stellar halo is dominated by the contributions of massive satellites, we show that satellites with low virial mass (i.e. low satellite-to-host virial mass ratio, VMR) are important contributors to the population of accreted GCs. The relative contribution of accretion events with low VMRs is highest for the halo population of blue GCs and gradually decreases for red GCs and accreted stars. As a consequence of the reduced efficiency of dynamical friction on minor mergers, our populations of cosmologically accreted blue GCs are systematically more spatially extended and have higher velocity dispersions than for red GCs, in agreement with observations. For the same reason, assembly histories including a higher fraction of minor mergers result in more spatially extended GC populations. GC line-of-sight velocity distributions featuring negative values of the kurtosis κ, as recently observed, are ubiquitous in our models. Therefore, κ < 0 is not at odds with an accretion scenario, and is in fact a fingerprint of the important contribution of minor mergers. However, our populations of accreted GCs remain mostly radially biased, with profiles of the anisotropy parameter β that are mildly radial in the centre [β (r < 10 kpc)∼ 0.2] and strongly radially anisotropic at large galactocentric distances [β (r > 30 kpc) ≳ 0.6], for both red and blue populations.