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Galactic halos of self-interacting dark matter

Steen Hannestad

TL;DR

The paper addresses the mismatch between CDM predictions and observed galactic dynamics by testing self-interacting dark matter (SIDM) as a remedy. It uses a simple Boltzmann-based framework with a velocity-independent cross section and a natural dividing scale $\sigma_0$ to simulate halo formation and evolution, solving the Boltzmann equation without cosmic expansion in a spherical, phase-space approach. The key findings show that strong SIDM yields shallower cores in halos, while intermediate cross sections can cause particle ejection on a timescale of order $t_{ m H}$, potentially preventing equilibrium; SIDM does not alter the initial power spectrum. This work supports SIDM as a viable modification to CDM on galactic scales and motivates more detailed N-body investigations to quantify its impact.

Abstract

Recent, very accurate simulations of galaxy formation have revealed that the standard cold dark matter model has great difficulty in explaining the detailed structure of galaxies. One of the major problems is that galactic halos are too centrally concentrated. Dark matter self-interactions have been proposed as a possible means of resolving this inconsistency. Here, we investigate quantitatively the effect of dark matter self interactions on formation of galactic halos. Our numerical framework is extremely simple, while still keeping the essential physics. We confirm that strongly self-interacting dark matter leads to less centrally concentrated structures. Interestingly, we find that for a range of different interaction strengths, the dark matter halos are unstable to particle ejection on a timescale comparable to the Hubble time.

Galactic halos of self-interacting dark matter

TL;DR

The paper addresses the mismatch between CDM predictions and observed galactic dynamics by testing self-interacting dark matter (SIDM) as a remedy. It uses a simple Boltzmann-based framework with a velocity-independent cross section and a natural dividing scale to simulate halo formation and evolution, solving the Boltzmann equation without cosmic expansion in a spherical, phase-space approach. The key findings show that strong SIDM yields shallower cores in halos, while intermediate cross sections can cause particle ejection on a timescale of order , potentially preventing equilibrium; SIDM does not alter the initial power spectrum. This work supports SIDM as a viable modification to CDM on galactic scales and motivates more detailed N-body investigations to quantify its impact.

Abstract

Recent, very accurate simulations of galaxy formation have revealed that the standard cold dark matter model has great difficulty in explaining the detailed structure of galaxies. One of the major problems is that galactic halos are too centrally concentrated. Dark matter self-interactions have been proposed as a possible means of resolving this inconsistency. Here, we investigate quantitatively the effect of dark matter self interactions on formation of galactic halos. Our numerical framework is extremely simple, while still keeping the essential physics. We confirm that strongly self-interacting dark matter leads to less centrally concentrated structures. Interestingly, we find that for a range of different interaction strengths, the dark matter halos are unstable to particle ejection on a timescale comparable to the Hubble time.

Paper Structure

This paper contains 7 sections, 11 equations, 3 figures.

Table of Contents

  1. introduction
  2. The physics of self-interacting dark matter
  3. Numerical scheme
  4. Results
  5. Collisionless systems
  6. Collisional systems
  7. Discussion

Figures (3)

  • Figure 1: The mass distribution, $M(r)$, for the collisionless dark matter collapse at three different times. The solid line is for $t=\tau$, the dashed for $t=10\tau$, and the dot-dashed for $t=20 \tau$.
  • Figure 2: The mass distribution as a function of $r$ at three different times and three different values of $\sigma$. The solid lines are for $t=\tau$, the long-dashed for $t=15\tau$, the dashed for $t=20 \tau$ and the dotted for $t=25 \tau$.
  • Figure 3: The rotational velocity at $t=20 \tau$ for three different values of $\sigma$. The crosses are the observational rotation curve for the low surface brightness galaxy UGC128 moore2.