The unrelaxed dynamical structure of the galaxy cluster Abell 85
  For the first time, we explore the dynamics of the central region of a galaxy
cluster within $r_{500}\sim 600h^{-1}$~kpc from its center by combining optical
and X-ray spectroscopy. We use (1) the caustic technique that identifies the
cluster substructures and their galaxy members with optical spectroscopic data,
and (2) the X-ray redshift fitting procedure that estimates the redshift
distribution of the intracluster medium (ICM). We use the spatial and redshift
distributions of the galaxies and of the X-ray emitting gas to associate the
optical substructures to the X-ray regions. When we apply this approach to
Abell 85 (A85), a complex dynamical structure of A85 emerges from our analysis:
a galaxy group, with redshift $z=0.0509 \pm 0.0021$ is passing through the
cluster center along the line of sight dragging part of the ICM present in the
cluster core; two additional groups, at redshift $z=0.0547 \pm 0.0022$ and
$z=0.0570 \pm 0.0020$, are going through the cluster in opposite directions,
almost perpendicularly to the line of sight, and have substantially perturbed
the dynamics of the ICM. An additional group in the outskirts of A85, at
redshift $z=0.0561 \pm 0.0023$, is associated to a secondary peak of the X-ray
emission, at redshift $z=0.0583^{+0.0039}_{-0.0047}$. Although our analysis and
results on A85 need to be confirmed by high-resolution spectroscopy, they
demonstrate how our new approach can be a powerful tool to constrain the
formation history of galaxy clusters by unveiling their central and surrounding
structures.
