A spectroscopic study of NGC 6251 and its companion galaxies

Measurements of the velocities of galaxies thought to be associated with the giant radio galaxy NGC 6251 confirm the presence of a poor cluster with a systemic redshift of z= 0.0244 +/- 0.0004 and a line-of-sight velocity dispersion of sigma_{z}= 283 (+109, -52) km/s. This suggests a cluster atmosphere temperature of T = 0.7 (+0.6, -0.2) keV, which is not enough to confine the radio jet by gas pressure. The core of NGC 6251 shows strong emission lines of [O III] and H alpha + [N II], but there is no evidence for line emission from the jet (detected in optical continuum by Keel (1988)).