  In this letter, we show that a massive black hole (MBH) which falls into the
center of a galaxy in dynamical timescale leaves a weak cusp ($\rho \propto
r^{-1/2}$) around it, which is in good agreement with the recent observations
of luminous ellipticals by Hubble Space Telescope. Such event is a natural
outcome of merging of two galaxies which have central MBHs. This is the only
known mechanism to form weak cusps in luminous ellipticals. Therefore, the
existence of the weak cusps indicates the central BHs of luminous ellipticals
have fallen to the center from outside, most likely during a major merger
event.
