  Scanning tunneling spectroscopic studies of Bi2Se3 epitaxial films on Si
(111) substrates reveal highly localized unitary impurity resonances associated
with non-magnetic quantum impurities. The strength of the resonances depends on
the energy difference between the Fermi level ({E_F}) and the Dirac point
({E_D}) and diverges as {E_F} approaches {E_D}. The Dirac-cone surface state of
the host recovers within ~ 2{\AA} spatial distance from impurities, suggesting
robust topological protection of the surface state of topological insulators
against high-density impurities that preserve time reversal symmetry.
