Spectroscopy of the NGC 4782/3 Dumbbell Dominated Group of Galaxies: Dark Haloes and Merger of Subgroups

A nearly complete sample of new velocities for 96 galaxies covering an area 1^deg^ x 1.25^deg^ centered on NGC 4782/3 are presented. Including data from the literature a total of 125 velocities are compiled. We deduce that 40 galaxies are likely members of the group centered on NGC 4782/3, more than doubling the original confirmed 13 members. Based on this extended velocity sample we analyze the velocity structure of this unusual group. A bi-modal velocity structure centered on each of the dumbbell components provides evidence for an ongoing collision of two subgroups associated to these components. In velocity space the richer subgroup is centered on the more massive NGC 4782. From the subgroup dynamics we derive approximate dynamical masses of 5 x 10^13^ M_sun_ and 1 x 10^13^ M_sun_ for the NGC 4782 and NGC 4783 subgroups, respectively. This leads to a group M/L ~ 300 (M/L)_sun_, implying that -90% of the group mass is in dark form, within subsystems associated to each subgroup. The colliding elliptical galaxies have massive haloes which should be important in shaping the outcome of the collision and merger process. In particular, haloes could change the interpretation of the tidal distortions and play a role in the shaping of the radio double bent jet structure of 3C 278.