Geometric Support for Dark Matter by an Unaligned Einstein Ring in Abell 3827

TL;DR

This study uses gravitational lensing in galaxy cluster Abell 3827 to test the viability of alternative gravity theories, finding that a dominant dark matter halo aligned with visible matter better reproduces the Einstein ring than visible matter alone.

Contribution

It demonstrates that reproducing the Einstein ring requires a dominant, smoothly oriented dark matter halo aligned with visible matter, challenging alternative gravity theories based solely on visible matter.

Findings

Visible matter alone cannot reproduce the Einstein ring.

A dominant dark matter halo aligned with visible matter reproduces the ring.

The Einstein ring constrains the distribution and orientation of dark matter.

Abstract

The non-detection of dark matter (DM) particles in increasingly stringent laboratory searches has encouraged alternative gravity theories where gravity is sourced only from visible matter. Here, we consider whether such theories can pass a two-dimensional test posed by gravitational lensing -- to reproduce a particularly detailed Einstein ring in the core of the galaxy cluster Abell 3827. We find that when we require the lensing mass distribution to strictly follow the shape (ellipticity and position angle) of the light distribution of cluster member galaxies, intracluster stars, and the X-ray emitting intracluster medium, we cannot reproduce the Einstein ring, despite allowing the mass-to-light ratios of these visible components to freely vary with radius to mimic alternative gravity theories. Alternatively, we show that the detailed features of the Einstein ring are accurately reproduced by allowing a smooth, freely oriented DM halo in the lens model, with relatively small contributions from the visible components at a level consistent with their observed brightnesses. This dominant DM component is constrained to have the same orientation as the light from the intracluster stars, indicating that the intracluster stars trace the gravitational potential of this component. The Einstein ring of Abell 3827 therefore presents a new challenge for alternative gravity theories: not only must such theories find agreement between the total lensing mass and visible mass, but they must also find agreement between the projected sky distribution of the lensing mass and that of the visible matter, a more stringent test than has hitherto been posed by lensing data.