In order to probe the inside-out formation of the most massive galaxies in
the Universe, we have explored the radial (0.1 < R < 8 kpc) variation of the
spectral energy distribution (SED) of M87 from UV to IR. For this purpose, we
have combined high resolution data in 16 different bands. Our analysis indicate
that the age of the stellar population of M87 remains almost unchanged with
radius. However, the metallicity ([Z/H]) profile presents three different
zones: the innermost kpc shows a plateau with supersolar metallicity, followed
by a decline in metallicity down to 5 kpc and another plateau afterwards. The
size of the inner plateau is similar to the expected size (Re) of an object
with the predicted mass of M87 at z=2. The global [Z/H] gradient is -0.26 +-
0.10, similar to those found in other nearby massive ellipticals. The observed
change in the stellar population of M87 is consistent with a rapid formation of
the central part (R<5 kpc) of this galaxy followed by the accretion of the
outer regions through the infall of more metal-poor material.