We present the analysis and results of a new VRI photometric and spectroscopic survey of the central \sim~ { } 1.8 { \times } 1.2 ~ { } { Mpc } ^ { 2 } region of the galaxy cluster A3921 ( z = 0.094 ) .
We detect the presence of two dominant clumps of galaxies with a mass ratio of \sim 5 : 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 .
The two sub-clusters are observed in the central phase of their merging process ( \pm 0.3 Gyr ) , with a collision axis nearly perpendicular to the line of sight .
Based on the spectral features of the galaxies belonging to A3921 , we estimate the star formation properties of the confirmed cluster members .
Substantial fractions of both emission-line ( \sim 13 % ) and post-star-forming objects ( k+a ’ s , \sim 16 % ) are detected , comparable to those measured at intermediate redshifts .
Our analysis reveals a lack of bright post-star forming objects in A3921 with respect to higher redshift clusters , while the fraction of k+a ’ s increases towards fainter magnitudes ( M _ { R _ { AB } } > -20 ) .
Similar results were obtained in Coma cluster by Poggianti et al . 2004 , but at still fainter magnitudes , suggesting that the maximum mass of actively star-forming galaxies increases with redshift ( “ downsizing effect ” ) .
The spatial and velocity distributions of k+a ’ s galaxies do not show significant differences to that of the passive population , and to the whole cluster .
Most of these objects show relatively red colours and moderate Balmer absorption lines , which suggest star formation has ceased \sim 1 - 1.5 Gyr ago .
Their presence is therefore difficult to relate to the on-going merging event .
We find that star forming galaxies share neither the same kinematics , nor the same projected distribution of the passive cluster members .
Moreover , most of emission-line galaxies are concentrated in A3921-B and in the region between the two sub-clusters .
We therefore suggest that the ongoing merger may have triggered the star-formation episode in at least a fraction of the observed emission-line galaxies .