The merging galaxy cluster A520 --- a broken-up cool core, a dark subcluster, and an X-ray channel

TL;DR

This study uses deep Chandra X-ray observations to analyze the complex gas dynamics, magnetic field effects, and dark substructures in the merging galaxy cluster A520, revealing suppressed thermal conductivity and a dark subcluster.

Contribution

It provides detailed high-resolution temperature maps and insights into magnetic suppression of thermal conduction, as well as identifying a dark subcluster and a unique X-ray channel in A520.

Findings

Cool core fragments are stripped and connected by magnetic fields.

Thermal conductivity is suppressed by over 100 times across magnetic fields.

Identification of a dark subcluster with very low gas mass to total mass ratio.

Abstract

We present results from a deep Chandra X-ray observation of a merging galaxy cluster A520. A high-resolution gas temperature map, after the subtraction of the cluster-scale emission, reveals a long trail of dense, cool clumps --- apparently the fragments of a cool core that has been completely stripped from the infalling subcluster by ram pressure. In this scenario, we can assume that the clumps are still connected by the magnetic field lines. The observed temperature variations imply that thermal conductivity is suppressed by a factor >100 across the presumed direction of the magnetic field (as found in other clusters), and is also suppressed -along- the field lines by a factor of several. Two massive clumps in the periphery of A520, visible in the weak lensing mass map and the X-ray image, have apparently been completely stripped of gas during the merger, but then re-accreted the surrounding high-entropy gas upon exit from the cluster. An X-ray hydrostatic mass estimate for one of the clumps (that has simple geometry) agrees with the lensing mass. Its current gas mass to total mass ratio is very low, 1.5-3%, which makes it a "dark subcluster". We also found a curious low X-ray brightness channel (likely a low-density sheet in projection) going across the cluster along the direction of an apparent secondary merger. The channel may be caused by plasma depletion in a region of an amplified magnetic field (with plasma $\beta\sim 10-20$). The shock in A520 will be studied in a separate paper.