Testing the rotation versus merger scenario in the galaxy cluster Abell
  2107
  We search for global rotation of the intracluster medium (ICM) in the galaxy
cluster Abell 2107, where previous studies have detected rotational motion in
the member galaxies with a high significance level. By fitting the centroid of
the iron $K_{\alpha}$ line complex at 6.7--6.9 keV rest frame in {\sl Chandra}
ACIS-I spectra, we identify the possible rotation axis with the line that
maximizes the difference between the emission-weighted spectroscopic redshift
measured in the two halves defined by the line itself. Then, we measure the
emission-weighted redshift in linear regions parallel to the preferred rotation
axis, and find a significant gradient as a function of the projected distance
from the rotation axis, compatible with a rotation pattern with maximum
tangential velocity ${\tt v}_{\rm max}=1380\pm 600$ km/s at a radius
$\lambda_0\sim 160$ kpc. This result, if interpreted in the framework of
hydrostatic equilibrium, as suggested by the regular morphology of Abell 2107,
would imply a large mass correction of the order of $\Delta M = (6 \pm 4)\times
10^{13} M_\odot$ at $\sim 160$ kpc, which is incompatible with the cluster
morphology itself. A more conservative interpretation may be provided by an
unnoticed off-center, head-on collision between two comparable halos. Our
analysis confirms the peculiar dynamical nature of the otherwise regular
cluster Abell 2107, but is not able to resolve the rotation vs merger scenario,
a science case that can be addressed by the next-generation X-ray facilities
carrying X-ray bolometers onboard.
