  The diffuse extended outer regions of galaxies are hard to study because they
are faint, with typical surface brightness of 1% of the dark night sky. We can
tackle this problem by using resolved star tracers which remain visible at
large distances from the galaxy centres. This article describes the use of
Planetary Nebulae as tracers and the calibration of their properties as
indicators of the star formation history, mean age and metallicity of the
parent stars in the Milky Way and Local Group galaxies . We then report on the
results from a deep, extended, planetary nebulae survey in a 0.5 sqdeg region
centred on the brightest cluster galaxy NGC 4486 (M87) in the Virgo cluster
core, carried out with SuprimeCam@Subaru and FLAMES-GIRAFFE@VLT. Two PN
populations are identified out to 150 kpc distance from the centre of M87. One
population is associated with the M87 halo and the second one with the
intracluster light in the Virgo cluster core. They have different line-of-sight
velocity and spatial distributions, as well as different planetary nebulae
specific frequencies and luminosity functions. The intracluster planetary
nebulae in the surveyed region correspond to a luminosity of four times the
luminosity of the Large Magellanic Cloud. The M87 halo planetary nebulae trace
an older, more metal-rich, parent stellar population. A substructure detected
in the projected phase-space of the line-of-sight velocity vs. major axis
distance for the M87 halo planetary nebulae provides evidence for the recent
accretion event of a satellite galaxy with luminosity twice that of M33. The
satellite stars were tidally stripped about 1 Gyr ago, and reached apocenter at
a major axis distance of 60-90 kpc from the centre of M87. The M87 halo is
still growing significantly at the distances where the substructure is
detected.
