Close-up view of an ongoing merger between the NGC 4839 group and the
  Coma cluster - a post-merger scenario
  We study a merger of the NGC 4839 group with the Coma cluster using X-ray
observations from the XMM-Newton and Chandra telescopes. X-ray data show two
prominent features: (i) a long (~600 kpc in projection) and bent tail of cool
gas trailing (towards south-west) the optical center of NGC 4839, and ii) a
"sheath" region of enhanced X-ray surface brightness enveloping the group,
which is due to hotter gas. While at first glance the X-ray images suggest that
we are witnessing the first infall of NGC 4839 into the Coma cluster core, we
argue that a post-merger scenario provides a better explanation of the observed
features and illustrate this with a series of numerical simulations. In this
scenario, the tail is formed when the group, initially moving to the
south-west, reverses its radial velocity after crossing the apocenter, the ram
pressure ceases and the ram-pressure-displaced gas falls back toward the center
of the group and overshoots it. Shortly after the apocenter passage, the
optical galaxy, dark matter and gaseous core move in a north-east direction,
while the displaced gas continues moving to the south-west. The "sheath" is
explained as being due to interaction of the re-infalling group with its own
tail of stripped gas mixed with the Coma gas. In this scenario, the shock,
driven by the group before reaching the apocenter, has already detached from
the group and would be located close to the famous relic to the south-west of
the Coma cluster.
