Spectra of single confirmed PNs. The top panel shows the spectrum for a PN dynamically bound to the halo component. The middle and bottom panels show spectra for PNs dynamically unbound to the halo (see Sect. 3), with higher (middle panel) and lower (bottom panel) velocity than the M 87 systemic velocity. Red vertical lines mark the positions of the two oxygen lines at their redshifted wavelengths. We smoothed the spectra to 0.015 nm per pixel.