For the first time , we explore the dynamics of the central region of a galaxy cluster within r _ { 500 } \sim 600 h ^ { -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 .