Structure of Abell 1995 from optical and X-ray data: a galaxy cluster
  with an elongated radio halo
  Abell 1995 is a puzzling galaxy cluster hosting a powerful radio halo, but it
has not yet been recognized as a obvious cluster merger, as usually expected
for clusters with diffuse radio emission. We aim at an exhaustive analysis of
the internal structure of Abell 1995 to verify if this cluster is really
dynamically relaxed, as reported in previous studies. We base our analysis on
new and archival spectroscopic and photometric data for 126 galaxies in the
field of Abell 1995. The study of the hot intracluster medium was performed on
X-ray archival data. Based on 87 fiducial cluster members, we have computed the
average cluster redshift <z>=0.322 and the global radial velocity dispersion
~1300 km/s. We detect two main optical subclusters separated by 1.5 arcmin that
cause the known NE-SW elongation of the galaxy distribution and a significant
velocity gradient in the same direction. As for the X-ray analysis, we confirm
that the intracluster medium is mildly elongated, but we also detect three
X-ray peaks. Two X-ray peaks are offset with respect to the two galaxy peaks
and lie between them, thus suggesting a bimodal merger caught in a phase of
post core-core passage. The third X-ray peak lies between the NE galaxy peak
and a third, minor galaxy peak suggesting a more complex merger. Simple
analytical arguments suggest a merging scenario for Abell 1995, where two main
subsystems are seen just after the collision with an intermediate projection
angle. The high mass of Abell 1995 and the evidence of merging suggest it is
not atypical among clusters with known radio halos. Interestingly, our findings
reinforce the previous evidence for the peculiar dichotomy between the dark
matter and galaxy distributions observed in this cluster.
