The planetary nebula population in the halo of M87

We investigate the diffuse light in the outer regions of the nearby elliptical galaxy M87 in the Virgo cluster, in the transition region between galaxy halo and intracluster light (ICL). Methods. The diffuse light is traced using planetary nebulas (PNs). The surveyed areas are imaged with a narrow-band filter centred on the redshifted [OIII].5007 angstrom emission line at the Virgo cluster distance (the on-band image) and with a broad-band V-filter (the off-band image). All PNs are identified through the on-off band technique using automatic selection criteria based on the distribution of the detected sources in the colour-magnitude diagram and the properties of their point-spread function. Results. We present the results of an imaging survey for PNs within a total effective area of 0.43 deg(2), covering the stellar halo of M87 up to a radial distance of 150 kpc. We extract a catalogue of 688 objects down to m5007 = 28.4, with an estimated residual contamination from foreground stars and background Lya galaxies, which amounts to similar to 35% of the sample. This is one of the largest extragalactic PN samples in number of candidates, magnitude depth, and radial extent, which allows us to carry out an unprecedented photometric study of the PN population in the outer regions of M87. We find that the logarithmic density profile of the PN distribution is shallower than the surface brightness profile at large radii. This behaviour is consistent with a model where the luminosity specific PN numbers for the M87 halo and ICL are different. Because of the depth of this survey we are also able to study the shape of the PN luminosity function (PNLF) in the outer regions of M87. We find a slope for the PNLF that is steeper at fainter magnitudes than the standard analytical PNLF formula and adopt a generalised model that treats the slope as a free parameter. Conclusions. The logarithmic PN number density profile is consistent with the superposition of two components associated with the halo of M87 and with the ICL, which have different a parameters. We derive a(2.5, halo) = (1.10+ 0.17 -0.21) x 10(-8) NPN L-1 (circle dot, bol) and a(2.5, ICL) = (3.29+ 0.60 -0.72) x 10(-8) NPN L-circle dot,bol(-1) for the halo and the intracluster stellar components, respectively. The fit of the generalised formula to the empirical PNLF for the M87 halo returns a value for the slope of 1.17 and a preliminary distance modulus to the M87 halo of 30.74. Comparing the PNLF of M87 and the M31 bulge, both normalised by the sampled luminosity, the M87 PNLF contains fewer bright PNs and has a steeper slope towards fainter magnitudes.