Tidal stripping and post-merger relaxation of dark matter haloes: causes and consequences of mass-loss

We study the properties of distinct dark matter haloes (i.e. those that are not subhaloes) that have a final virial mass M-vir at z = 0 less than their peak mass (M-peak) in the Bolshoi-Planck cosmological simulation. We identify two primary causes of halo mass-loss: relaxation after a major merger and tidal stripping by a massive neighbouring halo. Major mergers initially boost Mvir and typically cause the final halo to become more prolate and less relaxed and to have higher spin and lower NFW concentration. As the halo relaxes, high-energy material from the recent merger gradually escapes beyond the virial radius, temporarily resulting in a net negative accretion rate that reduces the halo mass by 5-15 per cent on average. Haloes that experience a major merger around z = 0.4 typically reach a minimum mass near z = 0. Tidal stripping mainly occurs in dense regions, and it causes haloes to become less prolate and have lower spins and higher NFW concentrations. Tidally stripped haloes often lose a large fraction of their peak mass (> 20 per cent) and most never recover (or even re-attain a positive accretion rate). Low-mass haloes can be strongly affected by both post-merger mass-loss and tidal stripping, while high-mass haloes are predominantly influenced by post-merger mass-loss and show few signs of significant tidal stripping.