Context :

Aims : To conduct an intensive study of the rich , X-ray luminous galaxy cluster Abell 773 at z = 0.22 containing a diffuse radio halo to determine its dynamical status .

Methods : Our analysis is based on new spectroscopic data obtained at the TNG telescope for 107 galaxies , 37 spectra recovered from the CFHT archive and new photometric data obtained at the Isaac Newton Telescope .
We use statistical tools to select 100 cluster members ( out to \sim 1.8 h _ { 70 } ^ { -1 } Mpc from the cluster centre ) , to analyse the kinematics of cluster galaxies and to determine the cluster structure .
Our analysis is also performed by using X-ray data stored in the Chandra archive .

Results : The 2D distribution shows two significant peaks separated by \sim 2 \mathrm { { } ^ { \prime } } in the EW direction with the main western one closely located at the position of the two dominant galaxies and the X-ray peak .
The velocity distribution of cluster galaxies shows two peaks at { v } \sim 65000 and \sim 67500 km s ^ { -1 } , corresponding to the velocities of the two dominant galaxies .
The low velocity structure has a high velocity dispersion – \sigma _ { v } = 800 - 1100 km s ^ { -1 } – and its galaxies are centred on the western 2D peak .
The high velocity structure has intermediate velocity dispersion – \sigma _ { v } \sim 500 km s ^ { -1 } – and is characterized by a complex 2D structure with a component centred on the western 2D peak and a component centred on the eastern 2D peak , these components probably being two independent groups .
We estimate a cluster mass within 1 h _ { 70 } ^ { -1 } Mpc of 6–11 \times 10 ^ { 14 } h _ { 70 } ^ { -1 } M _ { \odot } .
Our analysis of Chandra data shows the presence of two very close peaks in the core and the elongation of the X-ray emission in the NEE–SWW direction .

Conclusions : Our results suggest we are looking at probably two groups in an advanced stage of merging with a main cluster and having an impact velocity \Delta { { v } _ { rf } } \sim 2300 km s ^ { -1 } .
In particular , the radio halo seems to be related to the merger of the eastern group .