# Testing the rotation versus merger scenario in the galaxy cluster Abell 2107

## TL;DR

This study investigates the potential rotation of the intracluster medium in Abell 2107 using Chandra X-ray data, revealing a velocity gradient compatible with rotation but also consistent with a merger scenario, highlighting the cluster's complex dynamics.

## Contribution

The paper provides the first X-ray evidence of possible ICM rotation in Abell 2107 and discusses its implications for cluster mass estimates and dynamical state.

## Key findings

- Detected a significant velocity gradient consistent with rotation
- Estimated maximum tangential velocity of approximately 1380 km/s
- Results suggest either rotation or a merger scenario, but cannot conclusively distinguish between them.

## Abstract

We search for global rotation of the intracluster medium (ICM) in the galaxy cluster Abell 2107, where previous studies have detected rotational motion in the member galaxies with a high significance level. By fitting the centroid of the iron $K_{\alpha}$ line complex at 6.7--6.9 keV rest frame in {\sl Chandra} ACIS-I spectra, we identify the possible rotation axis with the line that maximizes the difference between the emission-weighted spectroscopic redshift measured in the two halves defined by the line itself. Then, we measure the emission-weighted redshift in linear regions parallel to the preferred rotation axis, and find a significant gradient as a function of the projected distance from the rotation axis, compatible with a rotation pattern with maximum tangential velocity ${\tt v}_{\rm max}=1380\pm 600$ km/s at a radius $\lambda_0\sim 160$ kpc. This result, if interpreted in the framework of hydrostatic equilibrium, as suggested by the regular morphology of Abell 2107, would imply a large mass correction of the order of $\Delta M = (6 \pm 4)\times 10^{13} M_\odot$ at $\sim 160$ kpc, which is incompatible with the cluster morphology itself. A more conservative interpretation may be provided by an unnoticed off-center, head-on collision between two comparable halos. Our analysis confirms the peculiar dynamical nature of the otherwise regular cluster Abell 2107, but is not able to resolve the rotation vs merger scenario, a science case that can be addressed by the next-generation X-ray facilities carrying X-ray bolometers onboard.