Modelling the effects of dark matter substructure on globular cluster evolution with the tidal approximation

We present direct N-body simulations of tidally filling 30 000 M-circle dot star clusters orbiting between 10 and 100 kpc in galaxies with a range of dark matter substructure properties. The time-dependent tidal force is determined based on the combined tidal tensor of the galaxy's smooth and clumpy dark matter components, the latter of which causes fluctuations in the tidal field that can heat clusters. The strength and duration of these fluctuations are sensitive to the local dark matter density, substructure fraction, sub-halo mass function, and the sub-halo mass-size relation. Based on the cold dark matter framework, we initially assume sub-haloes are Hernquist spheres following a power-lawmass function between 10(5) and 10(11) M-circle dot and find that tidal fluctuations are too weak and too short to affect star cluster evolution. Treating sub-haloes as point masses, to explore how denser sub-haloes affect clusters, we find that only sub-haloes with masses greater than 10(6) M-circle dot will cause cluster dissolution times to decrease. These interactions can also decrease the size of a cluster while increasing the velocity dispersion and tangential anisotropy in the outer regions via tidal heating. Hence increased fluctuations in the tidal tensor, especially fluctuations that are due to low-mass haloes, do not necessarily translate into mass-loss. We further conclude that the tidal approximation can be used to model cluster evolution in the tidal fields of cosmological simulations with a minimum cold dark matter sub-halo mass of 10(6) M-circle dot, as the effect of lower mass sub-haloes on star clusters is negligible.