XRISM Observations of The Prototypical Cold Front in Abell 3667

TL;DR

This study uses XRISM high-resolution X-ray spectroscopy to analyze the gas dynamics of the cold front in galaxy cluster Abell 3667, revealing velocity structures and turbulence that support an offset merger scenario and suggest magnetic stabilization.

Contribution

First high-resolution X-ray spectroscopic analysis of Abell 3667's cold front revealing detailed gas velocity and turbulence profiles, supporting a specific merger and magnetic stabilization model.

Findings

Gas in the core is blueshifted by ~200 km/s.

Cold front gas shows redshifted velocity of ~200 km/s.

Significant velocity change across the front (~535 km/s) indicating gas flow dynamics.

Abstract

We present high-resolution X-ray spectroscopy of the merging galaxy cluster Abell 3667 with \textit{XRISM}/Resolve. Two observations, targeting the cluster X-ray core and the prototypical cold front, were performed with exposures of 105 ks and 276 ks, respectively. We find that the gas in the core is blueshifted by $v_z\sim-200$ km s$^{-1}$ relative to the brightest cluster galaxy, while the low-entropy gas inside the cold front is redshifted by $v_z\sim 200$ km s$^{-1}$. As one moves further off-center across the front, the line-of-sight (LoS) velocity changes significantly, by $\Delta v_z=535^{+167}_{-154}$ km s$^{-1}$, back to the value similar to that in the core. There are no significant LoS velocity gradients perpendicular to the cluster symmetry axis. These features suggest that the gas forming the cold front is flowing in the plane oriented along the LoS, supporting an offset merger scenario in which the main cluster has passed in front of the subcluster and induced rotation of the core gas in the plane perpendicular to the sky. The region just inside the front exhibits the largest LoS velocity dispersion seen across two pointings, $\sigma_z\sim420$ km s$^{-1}$, which can be interpreted as a developing turbulence or a projection of the LoS velocity shear within the front. The large LoS velocity jump across the cold front, combined with the lack of Kelvin-Helmholtz instability on the surface of the front, suggests some mechanism to suppress it. For example, a magnetic field with $B>5\,\mu$G is required if the cold front is stabilized by magnetic draping.