  The measurability of multiple quasinormal (QN) modes, including overtones and
higher harmonics, with the Laser Interferometer Space Antenna is investigated
by computing the gravitational wave (GW) signal induced by an intermediate or
extreme mass ratio merger involving a supermassive black hole (SMBH). We
confirm that the ringdown of rapidly spinning black holes are long-lived, and
higher harmonics of the ringdown are significantly excited for mergers of small
mass ratios. We investigate the measurability and separability of the QN modes
for such mergers and demonstrate that the observation of GWs from rapidly
rotating SMBHs has an advantage for detecting superposed QN modes and testing
the no-hair theorem of black holes.
