Halo Properties in Cosmological Simulations of Self-Interacting Cold Dark Matter

TL;DR

This work addresses the long-standing tension between CDM predictions and galactic-scale observations by testing self-interacting dark matter (SIDM) with cross sections $\sigma_{\rm DM}$ in a cosmological N-body framework. Using a Monte Carlo collision scheme within a modified GADGET code, the authors compare CDM and SIDM halos across cross sections $\sigma_{\rm DM}=0, 10^{-24}, 10^{-23}\,{\rm cm^2}\,{\rm GeV}^{-1}$ in a $4\,h^{-1}{\rm Mpc}$ volume, resolving inner cores down to $\sim 1\,h^{-1}{\rm kpc}$. They find that SIDM yields larger cores, shallower inner slopes ($\alpha\approx -0.4$ to $-0.9$), central densities around $\rho_c\sim 0.01-0.02\,M_\odot{\rm pc}^{-3}$, and reduced mass concentrations $c_M$ relative to CDM, while halos remain non-isothermal and halos do not undergo core collapse at these cross sections. SIDM also produces more spherical halos, particularly in the inner regions, and modest suppression of the subhalo population, though robust statistics require higher-resolution simulations. Collectively, the results suggest $\sigma_{\rm DM}\approx 10^{-23}-10^{-24}\, {\rm cm^2}\,{\rm GeV}^{-1}$ provides a compelling fit to a broad range of observations, motivating further exploration across mass scales and improved subhalo modeling.

Abstract

We present a comparison of halo properties in cosmological simulations of collisionless cold dark matter (CDM) and self-interacting dark matter (SIDM) for a range of dark matter cross sections. We find, in agreement with various authors, that CDM yields cuspy halos that are too centrally concentrated as compared to observations. Conversely, SIDM simulations using a Monte Carlo N-body technique produce halos with significantly reduced central densities and flatter cores with increasing cross section. We introduce a concentration parameter based on enclosed mass that we expect will be straightforward to determine observationally, unlike that of Navarro, Frenk & White, and provide predictions for SIDM and CDM. SIDM also produces more spherical halos than CDM, providing possibly the strongest observational test of SIDM. We discuss our findings in relation to various relevant observations as well as SIDM simulations of other groups. Taking proper account of simulation limitations, we find that a dark matter cross section per unit mass of sigma_DM ~= 10^{-23}-10^{-24} cm^2/GeV is consistent with all current observational constraints.