Galaxies around the giant double radio source DA 240: Redshifts and the discovery of an unusual association

We have investigated galaxies near the giant radio source DA 240 in a study of the source's environment. Spectroscopic observations were obtained with the 2.16 m optical telescope of the National Astronomical Observatories of China. We have measured the redshifts of 26 objects, and find that 16 have nearly the same value as the DA 240 galaxy, and hence form a substantial group. Most of the others have lower redshifts and are probably outlying members of the Zwicky cluster Zw 0756.1+5616. We note that the radio galaxy host dominates the group. Ten of the galaxies in the group are aligned with the lobes of the radio source, and eight are projected on the components. We suggest that there may be a causal relationship, and find some evidence for enhanced activity among these galaxies. Moreover, there is a gradient in their velocities along the source axis. Whatever its explanation, we suggest that it establishes a dynamic timescale of some 10 super(9) yr. This may have been the time since the last merger, which triggered the giant radio source. The individual galaxy spectra are also presented and briefly discussed.