Multiple merging events in Abell 521

We present a detailed spatial and dynamical analysis of the central ∼2.2 h75 −1 Mpc region of the galaxy cluster Abell 521 (z = 0.247), based on 238 spectra (of which 191 new measurements) obtained at the 3.6 m Telescope of the European Southern Observatory and at the Canada-France-Hawaii Telescope. From the analysis of the 125 galaxies that are confirmed members of the cluster, we derive a location ("mean" velocity) of CBI = 74019 +112 −125 km s −1 and detect a complex velocity distri- bution with high velocity scale ("dispersion", S BI = 1325 +145 −100 km s −1 ), but clear departure from a single Gaussian component. When excluding a possible background group of four galaxies, the velocity dispersion remains still large (∼1200 km s −1 ). The general structure of the cluster follows a North-West/South-East direction, crossed by a perpendicular high density "ridge" of galaxies in the core region. The Northern region of the cluster is characterized by a lower velocity dispersion as compared to the whole cluster value; it hosts the BCG and a dynamically bound complex of galaxies, and it is associated with a group detected in X-ray (Arnaud et al. 2000). This region could be in a stage of pre-merger onto the main cluster. The small offset (∼+250 km s −1 ) in the mean velocity of the northern region as compared to the whole cluster suggests that the merging occurs partly in the plane of the sky. These results, taken together with the fact that most of the clumps detected on the isodensity maps, as well as the early-type galaxies and the brightest ones (L > L ∗ ) are aligned, suggest that this North-West/South-East direction is the preferred one for the formation of this cluster. The central high dense region ("ridge") shows a lower velocity location (CBI = 73625 +344 −350 km s −1 ) and significantly higher scale (1780 +234 −142 km s −1 ) as compared to the whole cluster values. This is due to the presence of a low-velocity group of galaxies with a high fraction of emission line objects. This can be explained in a scenario in which a merging of subclusters has recently occurred along the direction of the "ridge" with a significant compo- nent along the line of sight. The low-velocity group would then be a high-speed remnant of the collision which would have also triggered an episode of intense star formation responsible for the large fraction of late-type objects in this region.