A Joint Chandra and XMM-Newton View of Abell 3158: Massive Off-Centre
  Cool Gas Clump As A Robust Diagnostic of Merger Stage
  By analysing the Chandra and XMM-Newton archived data of the nearby galaxy
cluster Abell 3158, which was reported to possess a relatively regular, relaxed
morphology in the X-ray band in previous works, we identify a bow edge-shaped
discontinuity in the X-ray surface brightness distribution at about
$120h_{71}^{-1}$ kpc west of the X-ray peak. This feature is found to be
associated with a massive, off-centre cool gas clump, and actually forms the
west boundary of the cool clump. We find that the cool gas clump is moving at a
subsonic velocity of ~700 km/s toward west on the sky plane. We exclude the
possibility that this cool clump was formed by local inhomogeneous radiative
cooling in the intra-cluster medium, due to the effectiveness of the thermal
conduction on the time-scale of $\sim 0.3$ Gyr. Since no evidence for central
AGN activity has been found in Abell 3158, and this cool clump bears many
similarities to the off-centre cool gas clumps detected in other merging
clusters in terms of their mass, size, location, and thermal properties (e.g.
lower temperature and higher abundance as compared with the environment), we
speculate that the cool clump in Abell 3158 was caused by a merger event, and
is the remnant of the original central cool-core of the main cluster or the
infalling sub-cluster. This idea is supported not only by the study of
line-of-sight velocity distribution of the cluster member galaxies, but also by
the study of gas entropy-temperature correlation. This example shows that the
appearance of such massive, off-centre cool gas clumps can be used to diagnose
the dynamical state of a cluster, especially when prominent shocks and cold
fronts are absent.
