Contours: density of SDSS emission-line galaxies in the optical diagnostic diagrams (adapted from Kewley et al. 2006); only galaxies with lines measured with a signal-to-noise ratio 5 are included. Levels are in geometric sequence with a common ratio of 2. Galaxies below the curved solid lines are star-forming galaxies. In the left diagram, the dashed curve marks the transition from composite galaxies to AGN; in the middle and right panels, the oblique line separate LINER from Seyfert. The empty circles are the 3CR sources with z < 0.3, while the filled circles are the ELEGs (green) and the candidates (black). In the right diagram, the region bounded by the dotted lines defines the “relic neighbors” (see Sect. 5).