Earth-mass haloes and the emergence of NFW density profiles

TL;DR

This study uses simulations to show how earth-mass dark matter haloes evolve from steep to NFW-like profiles through mergers, highlighting the impact of initial conditions and merger history on halo structure.

Contribution

It demonstrates that NFW profiles emerge from initial steep profiles via mergers, emphasizing the role of early formation physics and merger history in halo profile development.

Findings

Initial haloes follow a ~r^{-1.5} profile.

Major mergers shallow the inner density profile.

NFW-like profiles are resilient to further mergers.

Abstract

We simulate neutralino dark matter ($\chi$DM) haloes from their initial collapse, at $\sim$ earth mass, up to a few percent solar. Our results confirm that the density profiles of the first haloes are described by a $\sim r^{-1.5}$ power-law. As haloes grow in mass, their density profiles evolve significantly. In the central regions, they become shallower and reach on average $\sim r^{-1}$, the asymptotic form of an NFW profile. Using non-cosmological controlled simulations, we observe that temporal variations in the gravitational potential caused by major mergers lead to a shallowing of the inner profile. This transformation is more significant for shallower initial profiles and for a higher number of merging systems. Depending on the merger details, the resulting profiles can be shallower or steeper than NFW in their inner regions. Interestingly, mergers have a much weaker effect when the profile is given by a broken power-law with an inner slope of $-1$ (such as NFW or Hernquist profiles). This offers an explanation for the emergence of NFW-like profiles: after their initial collapse, $r^{-1.5}$ $\chi$DM haloes suffer copious major mergers, which progressively shallows the profile. Once an NFW-like profile is established, subsequent merging do not change the profile anymore. This suggests that halo profiles are not universal but rather a combination of (1) the physics of the formation of the microhaloes and (2) their early merger history -- both set by the properties of the dark matter particle -- as well as (3) the resilience of NFW-like profiles to perturbations.