Massive early-type galaxies are now widely believed to have been assembled hierarchically through repeated mergers which provide a relatively constant flow of accreted mass over a long time. The innermost regions of massive galaxies appear to have formed the majority of their stars at high redshift and on short time-scales (e.g. Thomas et al. 2005) whereas their outer parts are likely assembled as a consequence of multiple major and minor merging (e.g. Trujillo, Ferreras & de La Rosa 2011). This two-phase formation picture (e.g. Naab, Johansson & Ostriker 2009) agrees with the observed size evolution of the massive galaxies. Atz ∼ 1 (2), massive early-type galaxies (M* ∼ 1011 M⊙) were a factor of 2 (4) smaller than present-day equal mass objects, having an average effective radii of only ∼1 kpc at z ∼ 2 (e.g. Daddi et al. 2005; Trujillo et al. 2006; Buitrago et al. 2008).
In order to probe this 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 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 anotherplateau 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.