A deep dive: Chandra observations of the NGC 4839 group falling into the Coma cluster

TL;DR

This paper presents deep Chandra X-ray observations of the NGC 4839 group merging into the Coma cluster, revealing detailed gas dynamics, turbulence, and thermal conduction suppression during the ongoing cluster growth.

Contribution

It provides the first detailed analysis of the gas features, turbulence, and conduction suppression in the infalling group using high-resolution X-ray data.

Findings

Detection of a cold front and bow shock with Mach number ~1.5.

Surface brightness fluctuation analysis shows turbulence with Mach 0.1-0.5.

Thermal conduction is suppressed, allowing long-lived small-scale turbulence.

Abstract

Cosmological simulations of structure formation predict that galaxy clusters continue to grow and evolve through ongoing mergers with group-scale systems. During these merging events, the ram pressure applied by the intracluster medium acts to strip the gas from the infalling groups, forming large tails of stripped gas, which eventually become part of the main cluster. In this work, we present a detailed analysis of our new deep Chandra observations of the NGC 4839 group falling into the nearby Coma cluster, providing a unique opportunity to explore the way galaxy clusters in the local universe continue to grow. Our analysis reveals a cold front feature at the leading head of the group, preceded by a bow shock of hot gas in front with a Mach number of $\sim\! 1.5$. The power spectrum of surface brightness fluctuations in the tail shows that the slope gets less steep as the distance from the leading head increases, changing from $-2.35_{-0.06}^{+0.07}$ at the inner part of the tail to $-1.37_{-0.07}^{+0.09}$ at the outermost part of the tail. These values are shallower than the slope of the Kolmogorov 2D power spectrum, indicating that thermal conduction is being suppressed throughout the tail, enabling long-lived small-scale turbulence, which would typically be washed out if thermal conduction was not inhibited. The characteristic amplitude of surface brightness fluctuations in the tail suggests a mild level of turbulence with a Mach number in the range of 0.1-0.5, agreeing with that found for the infalling group in Abell 2142.