The Tidal Origin of the Environment Dependence of Halo Assembly

We uncover the origin of the puzzling anti-correlation between the formation epoch of galactic dark-matter haloes and their environment density. This correlation has been revealed from cosmological N -body simulations and it is in conflict with the simple excursion-set model of halo clustering. Using similar simulations, we first quantify the straightforward association of an early formation epoch with a reduced mass growth rate at late times. We then find that the primary driver of suppressed growth, by accretion or mergers, is tidal effects dominated by a neighbouring massive halo. The tidal effects range from a slowdown of the assembly of haloes due to the shear along the large-scale filaments that feed the massive halo to actual mass loss in haloes that pass through the massive halo. Our results suggest that the dependence of formation epoch on environment density is a secondary effect induced by the enhanced density of haloes in filaments near massive haloes where the tides are strong. Our measures of assembly rate are particularly correlated with the tidal field at high redshifts z ∼ 1.