Sky positions of the spectroscopically confirmed halo (red crosses) and IC PNs (blue crosses). The green plus sign indicates the centre of M 87. The ellipses (dotted lines) trace the M 87 isophotes between photometric major axis radii and , for position angle PA = − 25.6°, from Kormendy et al. (2009). The solid squares depict the area covered by our narrowband imaging survey (Longobardi et al. 2013). North is up, East to the left.