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 |Mᵥᵢᵣ| at |z| = 0 less than their peak mass (⁠|Mₚₑₐₖ|⁠) 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.