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A Test of the Collisional Dark Matter Hypothesis from Cluster Lensing

Jordi Miralda-Escude

TL;DR

This paper tests the collisional dark matter hypothesis by linking microscopic self-interactions to macroscopic halo shapes. Using strong gravitational lensing of the cluster MS2137-23, it defines a collisional radius and demonstrates that the inner cluster halo remains elliptical, implying negligible self-interaction over the cluster's age. The resulting bound $s_x/m_x < 10^{-25.5}\ \mathrm{cm^2/GeV}$ argues against velocity-independent self-interactions as a universal fix to dwarf-core and satellite-count discrepancies, though velocity-dependent or more complex scenarios remain possible. Overall, cluster lensing provides a stringent, geometry-based probe of dark matter self-interaction.

Abstract

Spergel & Steinhardt proposed the possibility that the dark matter particles are self-interacting, as a solution to two discrepancies between the predictions of cold dark matter models and the observations: first, the observed dark matter distribution in some dwarf galaxies has large, constant-density cores, as opposed to the predicted central cusps; and second, small satellites of normal galaxies are much less abundant than predicted. The dark matter self-interaction would produce isothermal cores in halos, and would also expel the dark matter particles from dwarfs orbiting within large halos. However, another inevitable consequence of the model is that halos should become spherical once most particles have interacted. Here, I rule out this model by the fact that the innermost regions of dark matter halos in massive clusters of galaxies are elliptical, as shown by gravitational lensing and other observations. The absence of collisions in the lensing cores of massive clusters implies that any dark matter self-interaction is too weak to have affected the observed density profiles in the dark-matter dominated dwarf galaxies, or to have eased the destruction of dwarf satellites in galactic halos. If $s_x$ is the cross section and $m_x$ the mass of the dark matter particle, then $s_x/m_x < 10^{-25.5} \cm^2/\gev$.

A Test of the Collisional Dark Matter Hypothesis from Cluster Lensing

TL;DR

This paper tests the collisional dark matter hypothesis by linking microscopic self-interactions to macroscopic halo shapes. Using strong gravitational lensing of the cluster MS2137-23, it defines a collisional radius and demonstrates that the inner cluster halo remains elliptical, implying negligible self-interaction over the cluster's age. The resulting bound argues against velocity-independent self-interactions as a universal fix to dwarf-core and satellite-count discrepancies, though velocity-dependent or more complex scenarios remain possible. Overall, cluster lensing provides a stringent, geometry-based probe of dark matter self-interaction.

Abstract

Spergel & Steinhardt proposed the possibility that the dark matter particles are self-interacting, as a solution to two discrepancies between the predictions of cold dark matter models and the observations: first, the observed dark matter distribution in some dwarf galaxies has large, constant-density cores, as opposed to the predicted central cusps; and second, small satellites of normal galaxies are much less abundant than predicted. The dark matter self-interaction would produce isothermal cores in halos, and would also expel the dark matter particles from dwarfs orbiting within large halos. However, another inevitable consequence of the model is that halos should become spherical once most particles have interacted. Here, I rule out this model by the fact that the innermost regions of dark matter halos in massive clusters of galaxies are elliptical, as shown by gravitational lensing and other observations. The absence of collisions in the lensing cores of massive clusters implies that any dark matter self-interaction is too weak to have affected the observed density profiles in the dark-matter dominated dwarf galaxies, or to have eased the destruction of dwarf satellites in galactic halos. If is the cross section and the mass of the dark matter particle, then .

Paper Structure

This paper contains 4 sections, 14 equations, 1 figure.

Table of Contents

  1. Introduction
  2. The Collisional Radius in Dwarf Galaxies and in Galaxy Clusters
  3. The core of the cluster MS2137-23 is elliptical
  4. Discussion

Figures (1)

  • Figure 1: Schematic representation of the lensing configuration in the cluster MS2137-23 discussed in § 3. The lens center is at $C$, the source is located at $S$, and its two images are observed at $R$ (which is the image on the radial critical line) and $I$. The angle of misalignment $\gamma$ between the two images relative to the center would be zero for a spherical potential. The radial and azimuthal components of the deflection angles ($\alpha_{\theta}$ and $\alpha_{\phi}$) are indicated.