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 \overline { z } = 0.0244 \pm 0.0004 and a line-of-sight velocity dispersion of \sigma _ { z } = 283 ( +109 , -52 ) km s ^ { -1 } .
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 ) ) .