Tidal Effects and the Environment Dependence of Halo Assembly

TL;DR

This paper investigates how tidal forces from nearby massive haloes influence the formation and growth of dark-matter haloes, explaining the environment dependence and anti-correlation observed in cosmological simulations.

Contribution

It demonstrates that tidal effects are a primary driver of suppressed halo growth and environmental dependence, providing a physical explanation for the observed correlations.

Findings

Tidal effects cause mass loss and slow down halo assembly near massive haloes.

Halo formation epoch correlates with tidal field strength at high redshifts.

Environmental dependence is secondary, driven by tidal influences in filaments.

Abstract

We explore a possible origin for 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 is in conflict with the Extended Press-Schechter 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 a primary driver of suppressed growth, by accretion and 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. Using the restricted three-body problem, we show that haloes are prone to tidal mass loss within 1.5 virial radii of a larger 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.