Mergers of luminous early-type galaxies
  We have made a careful selection of a large complete volume-limited sample
(1209) of projected close pairs (7<r_p<50 kpc) of luminous early-type galaxies
(M_r<-21.5) in the local universe (z<0.12) from the SDSS data. 249 (21%) of
them show interaction features, which suggests that about 0.8% of the galaxies
are merging. We derived a comoving volume merger rate of ~(1.0+/-0.4)\times
10^{-5} Mpc^{-3} Gyr^{-1} for luminous early-type galaxies. This is a direct
observational determination of the merger rate of luminous galaxies in the
local universe. We also obtained the chirp mass distribution of supermassive
black hole (SMBH) binary following log[Phi(log M
/M_{odot})]=(21.7+/-4.2)-(3.0+/-0.5)log M/M_{odot}. With less assumptions than
previous works, we estimated the strain amplitude of the gravitational wave
(GW) background from coalescence of SMBH binaries at frequency 10^{-9}-10^{-7}
Hz.
