Spatial distribution of PN candidates (black crosses). Red crosses represent objects classified as spurious after visual inspection. The magenta cross indicates the centre of M 87. Dotted ellipses trace the M 87 isophotes from R = 2.8 to R = 40.7′ along the photometric major axis, at a position angle PA = −25.6° (Kormendy et al. 2009). The solid squares depict our survey area. North is up, east to the left.