  The central regions of galaxies show the presence of massive black holes
and/or dense stellar systems. The question about their modes of formation is
still under debate. A likely explanation of the formation of the central dense
stellar systems in both spiral and elliptical galaxies is based on the orbital
decay of massive globular clusters in the central region of galaxies due to
kinetic energy dissipation by dynamical friction. Their merging leads to the
formation of a nuclear star cluster, like that of the Milky Way, where a
massive black hole (Sgr A*) is also present. Actually, high precision N-body
simulations (Antonini, Capuzzo-Dolcetta et al. 2012, ApJ, 750, 111) show a good
fit to the observational characteristics of the Milky Way nuclear cluster,
giving further reliability to the cited `migratory' model for the formation of
compact systems in the inner galaxy regions.
