1. Dynamical state and star formation properties of the merging galaxy cluster Abell 3921 C. Ferrari 1,2, C. Benoist 1, S. Maurogordato 1, A. Cappi 3, E. Slezak 1 1 Observatoire de la Côte d’Azur, Nice, France - 2 Institut für Astrophysik, Innsbruck, Austria - 3 Osservatorio Astronomico di Bologna, Italy 15’15’ Abstract In the framework of the hierarchical model of structure formation, galaxy clusters form through accretion and merging of substructures of smaller mass. Merging clusters are a privileged laboratory to test the physics of evolutionary effects on their members. In particular, it is still debated how star formation is affected by the merging process (e.g. Evrard 1991, Fujita et al. 1999). In this context, after characterizing the dynamical state of the merging cluster Abell 3921, we analyze its star formation properties. Velocity distribution of the whole dataset Velocity distributions of A3921-A and of A3921-B Finding chart (27’  20’) of A3921 galaxies with measured redshifts R-Band image from ESO-WFI@2.2m Spectroscopy from ESO-EFOSC2@3.6m = k+a candidates (inversion of H-K lines) = confirmed k+a = emission line galaxies Dynamical State Optical Morphology + Cluster Kinematics + Presence of an extended hot region oriented parallel to the line joining the centers of A3921-A and A3921-B (XMM observations, Belsole et al. 2004)  The merger is in its central phase (0.0  0.3 Gyr)  Collision in the plane of the sky Projected Morphology A3921 is characterized by: 1) the presence of several substructures 2) a large scale bimodal morphology (A3921-A and A3921-B) 3) an offset of the BCG from the main density peak of the clump A  system out of dynamical equilibrium Star Formation Properties of Cluster Members A3921 galaxies divided in different spectral types (following Dressler et al. 1999): 1) k (passive, old population of stars) – 71 % 2) k+a (recent star formation) – 16 % 3) e (ongoing star formation) – 13 % - e-type galaxies share neither the same kinematics nor the same projected distribution than passive cluster members & they are concentrated in the collision region The star formation episode could be the result of an interaction with the ICM, connected with the dynamical state of the cluster - k+a galaxies show signature of an old star formation (1-1.5 Gyr) activity that can hardly be related to the on-going merger Ongoing spectroscopic follow-up up to the virial radius may clarify this point X-ray residuals after subtraction of a 2D-  model (Belsole et al. 2004) overlaid on the red sequence galaxy density map of the central part of A3921 (22  18 arcmin 2 ) A3921-B A3921-A Projected galaxy density map, built on the basis of a multi-scale approach and using red sequence galaxies, on a 34  34 arcmin 2 field centered on A3921 and for three magnitude cuts. The two main sub-clusters are indicated by rectangles. The white crosses indicate the positions of the four brightest cluster galaxies SubsampleNgalV mean (km/s)  v (km/s) Skewness (%) Kurtosis (%) Whole cluster 10428047  76831  90  10  10 A3921-A4128017  1501008  130> 10> 20 A3921-B2027920  87451  150< 10> 20 Cluster Kinematics 1) The kinematical properties of the whole sample and of the two subclusters do not show strong signatures of merging 2) The dynamics of the central regions of the two clumps appears to be relatively unaffected by the merging event  virial mass estimate: M A /M B  5 Properties of the radial velocity distribution and significance level of shape estimators for the whole cluster and the two sub-clusters The bulk of X-ray emission in A3921- B is offset towards SW from the main concentration of galaxies: A3921-B is tangentially traversing A3921-A along a SW/NE direction Velocity distributions of the different spectral types BCG