Joint Suzaku and Chandra observations of the MKW4 galaxy group out to the virial radius

TL;DR

This study combines Suzaku and Chandra X-ray observations to analyze the MKW4 galaxy group, revealing its entropy profile, baryon fraction, and symmetry, which inform our understanding of galaxy group formation and baryon loss.

Contribution

First joint Suzaku and Chandra observations of MKW4 mapping gas properties out to the virial radius, showing well-behaved entropy profiles and low azimuthal scatter indicating a relaxed system.

Findings

Entropy profile follows a power-law of r^{1.1}

Baryon fraction at R200 is 11%, below cosmic average

Low azimuthal scatter suggests spherical symmetry

Abstract

We present joint Suzaku and Chandra observations of MKW4. With a global temperature of 1.6 keV, MKW4 is one of the smallest galaxy groups that have been mapped in X-rays out to the virial radius. We measure its gas properties from its center to the virial radius in the north, east, and northeast directions. Its entropy profile follows a power-law of $\propto r^{1.1}$ between R$_{500}$ and R$_{200}$ in all directions, as expected from the purely gravitational structure formation model. The well-behaved entropy profiles at the outskirts of MKW4 disfavor the presence of gas clumping or thermal non-equilibrium between ions and electrons in this system. We measure an enclosed baryon fraction of 11% at R$_{200}$, remarkably smaller than the cosmic baryon fraction of 15%. We note that the enclosed gas fractions at R$_{200}$ are systematically smaller for groups than for clusters from existing studies in the literature. The low baryon fraction of galaxy groups, such as MKW4, suggests that their shallower gravitational potential well may make them more vulnerable to baryon losses due to AGN feedback or galactic winds. We find that the azimuthal scatter of various gas properties at the outskirts of MKW4 is significantly lower than in other systems, suggesting that MKW4 is a spherically symmetric and highly relaxed system.