The galaxy halo formation in the absence of violent relaxation and a universal density profile of the halo center

TL;DR

This paper proposes that moderate energy relaxation during dark matter halo formation leads to a universal, cored density profile that aligns with observations of some galaxies, challenging traditional N-body simulation results.

Contribution

It introduces a new model of halo formation assuming moderate energy relaxation, resulting in a universal density profile close to observed galaxy cores.

Findings

The profile is nearly independent of initial perturbation properties.

The central core size can be estimated and is consistent with some galaxy observations.

Cusp formation is unlikely under moderate relaxation assumptions.

Abstract

While N-body simulations testify for a cuspy profile of the central region of the dark matter haloes, observations favor a shallow, cored density profile of the central region of, at least, some spiral galaxies and dwarf spheroidals. We show that a central profile, very close to the observed one, inevitably forms in the center of dark matter haloes if we make a supposition about a moderate energy relaxation of the system during the halo formation. If we assume the energy exchange between dark matter particles during the halo collapse to be not too intensive, the profile is universal: it depends almost not at all on the properties of the initial perturbation and is very akin, but not identical, to the Einasto profile with small Einasto index $n\sim 0.5$. We estimate the size of the 'central core' of the distribution, i.e., the extent of the very central region with a respectively gentle profile, and show that the cusp formation is unlikely, even if the dark matter is cold. The obtained profile is in a good agreement with observational data for, at least, some types of galaxies, but clearly disagrees with N-body simulations.