Direct Evidence of a Major Merger in the Perseus Cluster

TL;DR

This study provides direct evidence of a major off-axis merger in the Perseus galaxy cluster through weak lensing and simulations, resolving longstanding questions about its dynamical state.

Contribution

The paper identifies a missing subcluster and a mass bridge in Perseus, confirming a major merger scenario with combined observational and simulation analysis.

Findings

Detection of a subcluster halo at >5σ significance

Identification of a mass bridge between main and sub clusters

Simulation reproduces observed cold front and mass bridge features

Abstract

Although the Perseus cluster has often been regarded as an archetypical relaxed galaxy cluster, several lines of evidence including ancient, large-scale cold fronts, asymmetric plasma morphology, filamentary galaxy distribution, etc., provide a conflicting view of its dynamical state, suggesting that the cluster might have experienced a major merger. However, the absence of a clear merging companion identified to date hampers our understanding of the evolutionary track of the Perseus cluster consistent with these observational features. In this paper, through careful weak lensing analysis, we successfully identified the missing subcluster halo ($M_{200}=1.70^{+0.73}_{-0.59}\times10^{14}~M_{\odot}$) at the >5$\sigma$ level centered on NGC1264, which is located ~430 kpc west of the Perseus main cluster core. Moreover, a significant ($>3\sigma$) mass bridge, which is also traced by the cluster member galaxies, is detected between the Perseus main and sub clusters, which serves as direct evidence of gravitational interaction. With idealized numerical simulations, we demonstrate that a ~3:1 off-axis major merger can create the cold front observed ~700 kpc east of the main cluster core and also generate the observed mass bridge through multiple core crossings. This discovery resolves the long-standing puzzle of Perseus' dynamical state.