Direct constraints on the dark matter self-interaction cross-section from the merging galaxy cluster 1E0657-56

TL;DR

The paper investigates whether dark matter exhibits self-interactions by exploiting the unique Bullet Cluster merger 1E0657--56. By integrating X-ray, optical, and weak-lensing maps, the authors derive three independent, conservative bounds on the self-interaction cross-section $\sigma/m$, with the strongest constraint $<1\ \mathrm{cm^2\,g^{-1}}$ arising from the subcluster’s mass-to-light ratio consistency. This limit challenges much of the $(0.5-5)\ \mathrm{cm^2\,g^{-1}}$ range proposed to explain galaxy-core mass profiles, though velocity-dependent cross-sections remain a possible reconciliation. The authors note the result is an order-of-magnitude estimate and would benefit from detailed hydrodynamic simulations to potentially tighten the constraint.

Abstract

We compare new maps of the hot gas, dark matter, and galaxies for 1E0657-56, a cluster with a rare, high-velocity merger occurring nearly in the plane of the sky. The X-ray observations reveal a bullet-like gas subcluster just exiting the collision site. A prominent bow shock gives an estimate of the subcluster velocity, 4500 km/s, which lies mostly in the plane of the sky. The optical image shows that the gas lags behind the subcluster galaxies. The weak-lensing mass map reveals a dark matter clump lying ahead of the collisional gas bullet, but coincident with the effectively collisionless galaxies. From these observations, one can directly estimate the cross-section of the dark matter self-interaction. That the dark matter is not fluid-like is seen directly in the X-ray -- lensing mass overlay; more quantitative limits can be derived from three simple independent arguments. The most sensitive constraint, sigma/m<1 cm^2/g, comes from the consistency of the subcluster mass-to-light ratio with the main cluster (and universal) value, which rules out a significant mass loss due to dark matter particle collisions. This limit excludes most of the 0.5-5 cm^2/g interval proposed to explain the flat mass profiles in galaxies. Our result is only an order-of-magnitude estimate which involves a number of simplifying, but always conservative, assumptions; stronger constraints may be derived using hydrodynamic simulations of this cluster.

Figures (2)

• Figure 1: ( a) Overlay of the weak lensing mass contours on the optical image of 1E 0657--56. Dashed contours are negative (relative to an arbitrary zero level). The subcluster's DM peak is coincident within uncertainties with the centroid of the galaxy concentration. ( b) Overlay of the mass contours on the X-ray image (only the upper 4 of those in panel a are shown for clarity). The gas bullet lags behind the DM subcluster.
• Figure 2: Collision of two equal-mass particles in the subcluster reference frame.