Non-universality of dark-matter halos: cusps, cores, and the central potential

TL;DR

This paper demonstrates that the DARKexp model inherently predicts the non-universal density profiles of dark-matter halos, matching simulations and observations, and explains the variation in their core structures and shape parameters.

Contribution

It introduces DARKexp as a unifying theoretical model that accounts for the diversity in dark-matter halo profiles across different mass scales.

Findings

DARKexp fits the dispersion in simulated halo profiles.

DARKexp accommodates cored profiles in dwarf galaxies.

Analytical relations connect DARKexp parameters with other models.

Abstract

Dark-matter halos grown in cosmological simulations appear to have central NFW-like density cusps with mean values of $d\log\rho/d\log r \approx -1$, and some dispersion, which is generally parametrized by the varying index $\alpha$ in the Einasto density profile fitting function. Non-universality in profile shapes is also seen in observed galaxy clusters and possibly dwarf galaxies. Here we show that non-universality, at any given mass scale, is an intrinsic property of DARKexp, a theoretically derived model for collisionless self-gravitating systems. We demonstrate that DARKexp - which has only one shape parameter, $\phi_0$ - fits the dispersion in profile shapes of massive simulated halos as well as observed clusters very well. DARKexp also allows for cored dark-matter profiles, such as those found for dwarf spheroidal galaxies. We provide approximate analytical relations between DARKexp $\phi_0$, Einasto $\alpha$, or the central logarithmic slope in the Dehnen-Tremaine analytical $\gamma$-models. The range in halo parameters reflects a substantial variation in the binding energies per unit mass of dark-matter halos.