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.

中文翻译：

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朱雀和钱德拉联合观测 MKW4 星系群到维里半径

我们展示了 MKW4 的朱雀和钱德拉联合观测。MKW4 的全球温度为 1.6 keV，是在 X 射线中映射到维里半径的最小星系群之一。我们从其中心到北、东和东北方向的维里半径测量其气体性质。正如纯引力结构形成模型所预期的那样，它的熵分布在各个方向上都遵循 R$_{500}$ 和 R$_{200}$ 之间的 $\propto r^{1.1}$ 幂律。MKW4 外围的良好熵分布不利于该系统中离子和电子之间存在气体团块或热非平衡。我们在 R$_{200}$ 处测量了 11% 的封闭重子分数，明显小于 15% 的宇宙重子分数。我们注意到，在 R$_{200}$ 处的封闭气体分数对于组而言比文献中现有研究中的簇要小。星系群的低重子分数，例如 MKW4，表明它们较浅的引力势井可能使它们更容易受到 AGN 反馈或银河风造成的重子损失。我们发现MKW4外围各种气体性质的方位散射明显低于其他系统，表明MKW4是一个球对称和高度弛豫的系统。