Constraints on the Self-Interaction Cross-Section of Dark Matter from Numerical Simulations of the Merging Galaxy Cluster 1E 0657-5

TL;DR

This work tests the SIDM hypothesis in the 1E 0657-56 bullet cluster by combining high-quality X-ray and lensing data with controlled N-body simulations that include elastic, isotropic DM scattering. Through two independent constraints—the absence of a significant offset between the subcluster’s total mass and its galaxies, and the preservation of the subcluster’s mass-to-light ratio—the authors derive upper limits of $\\sigma/m < 1.25\\,\\mathrm{cm^2\\,g^{-1}}$ and $\\sigma/m < 0.7\\,\\mathrm{cm^2\\,g^{-1}}$ (the latter assuming equal pre-merger $M/L$). These results slightly improve upon previous analytic bounds and rule out most of the $0.5$–$5\\,\\mathrm{cm^2\\,g^{-1}}$ range for velocity-independent SIDM, indicating DM self-interactions are not strong in cluster environments. The findings bolster the collisionless CDM picture at large scales while informing SIDM models of particle physics and their velocity dependence, with implications for small-scale structure issues.

Abstract

(Abridged) We compare recent results from X-ray, strong lensing, weak lensing, and optical observations with numerical simulations of the merging galaxy cluster 1E0657-56. X-ray observations reveal a bullet-like subcluster with a prominent bow shock, while lensing results show that the positions of the total mass peaks are consistent with the centroids of the collisionless galaxies (and inconsistent with the X-ray brightness peaks). Previous studies, based on older observational datasets, have placed upper limits on the self-interaction cross-section of dark matter per unit mass, sigma/m, using simplified analytic techniques. In this work, we take advantage of new, higher-quality observational datasets by running N-body simulations of 1E0657-56 that include the effects of self-interacting dark matter, and comparing the results with observations. Furthermore, the recent data allow for a new independent method of constraining sigma/m, based on the non-observation of an offset between the bullet subcluster mass peak and galaxy centroid. This new method places an upper limit (68% confidence) of sigma/m < 1.25 cm^2/g. If we make the assumption that the subcluster and the main cluster had equal mass-to-light ratios prior to the merger, we derive our most stringent constraint of sigma/m < 0.7 cm^2/g, which comes from the consistency of the subcluster's observed mass-to-light ratio with the main cluster's, and with the universal cluster value, ruling out the possibility of a large fraction of dark matter particles being scattered away due to collisions. Our limit is a slight improvement over the previous result from analytic estimates, and rules out most of the 0.5 - 5cm^2/g range invoked to explain inconsistencies between the standard collisionless cold dark matter model and observations.

Figures (5)

• Figure 1: X-ray image with weak lensing mass contours overlain. The gas bullet lags the subcluster DM halo. The current separation of the subcluster and main cluster mass peaks is 720 kpc.
• Figure 2: Close up of the subcluster bullet region, with the DM (blue) and galaxy (red) centroid error contours overlain. The contours show the 68.3% and 99.7% error regions. The left panel shows the X-ray Chandra image, while the right shows the optical HST image.
• Figure 3: Density profile of an isolated King model cluster at $t=0$ (solid line), and after evolving for 1 Gyr with $\sigma/m =0$ (dotted line) and $\sigma/m = 0.7$ cm$^2$ g$^{-1}$ (dashed line).
• Figure 4: Total mass within projected radius $x$ for the cluster plotted in Figure \ref{['fig:rhor']}. Line-type indications are the same as in Figure \ref{['fig:rhor']}.
• Figure 5: The dependence of the subcluster galaxy and total mass centroid offset ($\Delta$ x, solid line) and the fractional change in the subcluster $M/L$ ratio ($f$, dashed line) on $\sigma/m$. Based on the values given in Table \ref{['tab:obscond']}.