X-Ray observations of a subhalo associated with the NGC 4839 group infalling toward the Coma cluster

TL;DR

This study uses Suzaku X-ray observations to analyze a dark subhalo associated with NGC 4839 in the Coma cluster, revealing its gas properties, morphology, and the effects of ram pressure stripping during infall.

Contribution

It provides detailed measurements of the subhalo's gas distribution, temperature, and pressure, and estimates the mass loss due to ram pressure and Kelvin-Helmholtz instabilities, offering new insights into subhalo evolution.

Findings

The subhalo's temperature beyond the truncation radius is 8-10 keV.

The gas fraction within the truncation radius is about 1/5 of regular groups.

Estimated gas mass loss due to ram pressure is approximately 3×10^11 solar masses.

Abstract

We report $Suzaku$ X-ray observations of the dark subhalo associated with the merging group of NGC 4839 in the Coma cluster. The X-ray image exhibits an elongated tail toward the southwest. The X-ray peak shifts approximately $1'$ away from the weak-lensing mass center toward the opposite direction of the Coma cluster center. We investigated the temperature, normalization, pressure, and entropy distributions around the subhalo. Excluding the X-ray tail, the temperature beyond the truncation radius is 8-10$~\rm keV$, which is two times higher than that of the subhalo and the X-ray tail. The pressure is nearly uniform excluding southern part of the subhalo at two times of the truncation radius. We computed the gas mass within the truncation radius and the X-ray tail. While the gas fraction within the truncation radius is about 5 times smaller than that of regular groups, the gas mass in the subhalo and the X-ray tail to weak-lensing mass ratio is consistent with that of regular groups. Assuming the infall velocity, $2000~\rm km~s^{-1}$, the ram pressure is 1.4 times greater than gravitational force per unit area. Assuming the Kelvin-Helmholtz instabilities, the total lost mass is approximately $3\times10^{11}~M_{\odot}$. If this gas had originally been within the truncation radius, the gas mass fraction of the subhalo would have been comparable with those of regular groups before infalling to the Coma cluster.