Context : The mechanisms producing the diffuse radio emission in galaxy clusters , and in particular their connection with cluster mergers , are still debated .

Aims : We seek to explore the internal dynamics of the cluster Abell 1758N , which has been shown to host a radio halo and two relics , and is known to be a merging bimodal cluster .

Methods : Our analysis is mainly based on new redshift data for 137 galaxies acquired at the Telescopio Nazionale Galileo , only four of which have redshifts previously listed in the literature .
We also used photometric data from the Sloan Digital Sky Survey and from the Canada–France–Hawaii Telescope archive .
We combined galaxy velocities and positions to select 92 cluster galaxies and analyzed the internal cluster dynamics .

Results : We estimate a cluster redshift of \left < z \right > = 0.2782 and quite a high line–of–sight ( LOS ) velocity dispersion \sigma _ { V } \sim 1300 km s ^ { -1 } .
Our 2D analysis confirms the presence of a bimodal structure along the NW–SE direction .
We add several pieces of information to the previous merging scenario : the two subclusters ( here A1758N ( NW ) and A1758N ( SE ) ) can not be separated in the velocity analyses and we deduce a small LOS velocity difference ( \Delta V _ { rf,LOS } \raise - 2.0 pt \hbox { \hbox to 0.0 pt { \hbox { $ \sim$ } } \raise 5.0 % pt \hbox { $ < $ } } 300 km s ^ { -1 } in the cluster rest–frame ) .
The velocity information successfully shows that A1758N is surrounded by two small groups and active galaxies infalling onto , or escaping from , the cluster .
Removing the two groups , we estimate \sigma _ { V,NW } \sim 1000 km s ^ { -1 } and \sigma _ { V,SE } \sim 800 km s ^ { -1 } for A1758N ( NW ) and A1758N ( SE ) , respectively .
We find that Abell 1758N is a very massive cluster with a range of M = 2 - 3 \times 10 ^ { 15 } h _ { 70 } ^ { -1 } M _ { \odot } , depending on the adopted model .

Conclusions : As expected for clusters that host powerful , extended , diffuse radio emissions , Abell 1758N is a major cluster merger just forming a massive system .