Creation of cosmic structure in the complex galaxy cluster merger Abell 2744

TL;DR

This paper provides a detailed multi-wavelength analysis of the complex merging galaxy cluster Abell 2744, revealing substructures, mass distribution, and dark matter properties, with implications for understanding cluster mergers and dark matter interactions.

Contribution

It offers the most detailed mass map of Abell 2744 using combined lensing and X-ray data, and introduces a new constraint on dark matter self-interaction cross section.

Findings

Identification of multiple substructures including 'dark', 'ghost', 'bullet', and 'stripped' components.

Support for a post-merger scenario similar to the Bullet Cluster.

Discovery of large separations between gas, dark matter, and galaxies, including a 'ghost' clump leading the dark matter by over 150 kpc.

Abstract

We present a detailed strong lensing, weak lensing and X-ray analysis of Abell 2744 (z = 0.308), one of the most actively merging galaxy clusters known. It appears to have unleashed `dark', `ghost', `bullet' and `stripped' substructures, each ~10^14 solar masses. The phenomenology is complex and will present a challenge for numerical simulations to reproduce. With new, multiband HST imaging, we identify 34 strongly-lensed images of 11 galaxies around the massive Southern `core'. Combining this with weak lensing data from HST, VLT and Subaru, we produce the most detailed mass map of this cluster to date. We also perform an independent analysis of archival Chandra X-ray imaging. Our analyses support a recent claim that the Southern core and Northwestern substructure are post-merger and exhibit morphology similar to the Bullet Cluster viewed from an angle. From the separation between X-ray emitting gas and lensing mass in the Southern core, we derive a new and independent constraint on the self-interaction cross section of dark matter particles sigma/m <~ 3 \pm 1 cm^2 g^-1. In the Northwestern substructure, the gas, dark matter, and galaxy components have become separated by much larger distances. Most curiously, the `ghost' clump (primarily gas) leads the `dark' clump (primarily dark matter) by more than 150 kpc. We propose an enhanced `ram-pressure slingshot' scenario which may have yielded this reversal of components with such a large separation, but needs further confirmation by follow-up observations and numerical simulations. A secondary merger involves a second `bullet' clump in the North and an extremely `stripped' clump to the West. The latter appears to exhibit the largest separation between dark matter and X-ray emitting baryons detected to date in our sky.