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