The central cusps in dark matter halos: fact or fiction?

TL;DR

This paper challenges the traditional view that central cusps in dark matter halos are physical, showing instead that they result from numerical effects in N-body simulations, specifically related to the tree algorithm used.

Contribution

It demonstrates that core formation in simulated dark matter halos is due to the properties of the tree algorithm, not physical relaxation, questioning the reliability criteria of such simulations.

Findings

Core formation is linked to the tree algorithm, not collisional relaxation.

Numerical 'violent relaxation' may cause artificial cusp formation.

The core-cusp problem might be a simulation artifact rather than a physical issue.

Abstract

We investigate the reliability of standard N-body simulations by modelling of the well-known Hernquist halo with the help of \texttt{GADGET-2} code (which uses the tree algorithm to calculate the gravitational force) and \texttt{ph4} code (which uses the direct summation). Comparing the results, we find that the core formation in the halo center (which is conventionally considered as the first sign of numerical effects, to be specific, of the collisional relaxation) has nothing to do with the collisional relaxation, being defined by the properties of the tree algorithm. This result casts doubts on the universally adopted criteria of the simulation reliability in the halo center. Though we use a halo model, which is theoretically proved to be stationary and stable, a sort of numerical 'violent relaxation' occurs. Its properties suggest that this effect is highly likely responsible for the central cusp formation in cosmological modelling of the large-scale structure, and then the 'core-cusp problem' is no more than a technical problem of N-body simulations.