Concentrations of Dark Haloes Emerge from Their Merger Histories
  The concentration parameter is a key characteristic of a dark matter halo
that conveniently connects the halo's present-day structure with its assembly
history. Using 'Dark Sky', a suite of cosmological $N$-body simulations, we
investigate how halo concentration evolves with time and emerges from the mass
assembly history. We also explore the origin of the scatter in the relation
between concentration and assembly history. We show that the evolution of halo
concentration has two primary modes: (1) smooth increase due to
pseudo-evolution; and (2) intense responses to physical merger events. Merger
events induce lasting and substantial changes in halo structures, and we
observe a universal response in the concentration parameter. We argue that
merger events are a major contributor to the uncertainty in halo concentration
at fixed halo mass and formation time. In fact, even haloes that are typically
classified as having quiescent formation histories experience multiple minor
mergers. These minor mergers drive small deviations from pseudo-evolution,
which cause fluctuations in the concentration parameters and result in
effectively irreducible scatter in the relation between concentration and
assembly history. Hence, caution should be taken when using present-day halo
concentration parameter as a proxy for the halo assembly history, especially if
the recent merger history is unknown.
