X-Ray Shapes of Elliptical Galaxies and Implications for Self-Interacting Dark Matter

TL;DR

This study measures the shapes of dark matter halos in elliptical galaxies using X-ray data to test models of self-interacting dark matter, finding results compatible with both self-interaction and cold dark matter scenarios.

Contribution

First to analyze X-ray shapes of elliptical galaxies to constrain dark matter self-interaction strength and compare with theoretical models.

Findings

Dark matter halos have ellipticities around 0.2-0.5.

Results are consistent with self-interacting dark matter at σ/m ~ 1 cm²/g.

Findings also compatible with cold dark matter scenarios.

Abstract

Several proposed models for dark matter posit the existence of self-interaction processes that can impact the shape of dark matter halos, making them more spherical than the ellipsoidal halos of collisionless dark matter. One method of probing the halo shapes, and thus the strength of the dark matter self-interaction, is by measuring the shape of the X-ray gas that traces the gravitational potential in relaxed elliptical galaxies. In this work we identify a sample of 11 relaxed, isolated elliptical galaxies and measure the ellipticity of the gravitating matter using X-ray images from the XMM-Newton and Chandra telescopes. We explore a variety of different mass configurations and find that the dark matter halos of these galaxies have ellipticities around $\epsilon\approx 0.2-0.5$. While we find non-negligible scatter in the ellipticity distribution, our results are consistent with some degree of self-interaction at the scale of $\sigma/m \sim 1$ cm$^2$/g, yet they also remain compatible with a cold dark matter scenario. We additionally demonstrate how our results can be used to directly constrain specific dark matter models and discuss implications for current and future simulations of self-interacting dark matter models.