A Closer Look at the Dynamical State of the High-redshift Galaxy Cluster SPT-CL J2215-3537

TL;DR

This paper reanalyzes the high-redshift galaxy cluster SPT-CL J2215-3537, revealing signs of dynamical activity and cold fronts, challenging previous assumptions of relaxation, and discussing implications for morphological classification biases.

Contribution

It provides the first detection of cold fronts at z > 1 and discusses how morphological parameters can bias dynamical state assessments at high redshift.

Findings

Detection of temperature anisotropies and cold fronts at z > 1.

Identification of a ghost cavity with significant mechanical power.

Evidence that the cluster is in a transient phase before a cooling-feedback cycle.

Abstract

We present a comprehensive reanalysis of the dynamical state of the high-redshift galaxy cluster SPT-CL J2215-3537 (z = 1.16), using the full set of available Chandra observations to characterize the thermodynamic and morphological properties of the intracluster medium. Although previously identified as one of the most distant dynamically relaxed systems based on X-ray morphological statistics, we find compelling evidence that SPT-CL J2215-3537 displays some level of dynamical activity. This includes temperature anisotropies consistent with the first detection of a pair of core-sloshing cold fronts at z > 1. We identify a ghost cavity candidate and estimate its mechanical power as log10(Pcav/10^42 erg s^-1) = 2.66 +- 0.23, confirming that radiative cooling strongly exceeds active galactic nucleus feedback heating. We show that SPT-CL J2215-3537 is likely in a short transient phase preceding the onset of a self-regulated cooling-feedback cycle. We recalculate traditional X-ray morphological parameters and discuss how non-self-similar evolution of parameters sensitive to the surface brightness cuspiness can bias dynamical classifications at high redshift.