Amplitude of Civ  in the 4DE1 optical plane (FWHM Hβ_BC vs. R_FeII). Population A and B sources are marked as circles and squares, respectively. Colors represent shift amplitudes. Blue: large Civ blueshifts (>− 1000 km s^-1), aquamarine: significant blueshifts ( = − 200 to − 1000 km s^-1); gray: no significant shift (|| < 200 km s^-1). Red and pink, large and significant   redshift, respectively. Left: 4DE1 plane for the RQ LOWZ sample. Right: 4DE1 plane for HE sample. The horizontal filled lines mark the minimum FWHM Hβ for virialized systems at log L = 44 and 47 (no sources may exist in the shaded area, Marziani et al. 2009).