Age and metallicity gradients in fossil ellipticals

Fossil galaxy groups are speculated to be old and highly evolved systems of galaxies that formed early in the universe and had enough time to deplete their $L^*$ galaxies through successive mergers of member galaxies, building up one massive central elliptical, but retaining the group X-ray halo. Considering that fossils are the remnants of mergers in ordinary groups, the merger history of the progenitor group is expected to be imprinted in the fossil central galaxy (FCG). We present for the first time radial gradients of single-stellar population (SSP) ages and metallicites in a sample of FCGs to constrain their formation scenario. Our sample comprises some of the most massive galaxies in the universe exhibiting an average central velocity dispersion of $σ_0=271±28$ km s$^-1$. Metallicity gradients are throughout negative with comparatively flat slopes of $∇_[Fe/H]=- 0.19±0.08$ while age gradients are found to be insignificant ($∇_age=0.00±0.05$). All FCGs lie on the fundamental plane, suggesting that they are virialised systems. We find that gradient strengths and central metallicities are similar to those found in cluster ellipticals of similar mass. The comparatively flat metallicity gradients with respect to those predicted by monolithic collapse ($∇_Z=-0.5$) suggest that fossils are indeed the result of multiple major mergers. Hence we conclude that fossils are not 'failed groups' that formed with a top heavy luminosity function. The low scatter of gradient slopes suggests a similar merging history for all galaxies in our sample.