Dynamical state and star formation properties of the merging galaxy cluster Abell 3921

We present the results of a new spectroscopic and photometric survey of the central region of the galaxy cluster A3921 (z=0.094). We detect the presence of two dominant clumps of galaxies: a main cluster centred on the BCG (A3921-A), and a NW sub-cluster (A3921-B) hosting the second brightest cluster galaxy. The distorted morphology of the two sub-clusters suggests that they are interacting, while the velocity distribution of 104 confirmed cluster members does not reveal strong signatures of merging. By applying a two-body dynamical formalism to the two sub-clusters of A3921, and by comparing our optical results to the X-ray analysis of A3921 based on XMM observations (Belsole et al. 2004), we conclude that A3921-B is probably tangentially traversing the main cluster along a SW/NE direction. Our estimate of the star formation properties of the cluster members reveals substantial fractions of both emission-line (~13%) and post-star-forming objects (k+a's,~16%). A lack of bright k+a's with respect to higher redshift clusters is observed, while the fraction of k+a's increases towards fainter magnitudes. The spatial and velocity distributions of k+a's galaxies, their red colours and their moderate Balmer absorption lines suggest that their presence is difficult to relate to the on-going merging event. Star forming galaxies, which are mostly concentrated in A3921-B and between the two sub-clusters, share neither the same kinematics, nor the same projected distribution of the passive cluster members. Their spectral properties may be at least partly realted to the ongoing merger.