Gravitational waves from supermassive black hole coalescence in a hierarchical galaxy formation model

We investigate the expected gravitational wave emission from coalescing supermassive black hole (SMBH) binaries resulting from mergers of their host galaxies. When galaxies merge, the SMBHs in the host galaxies sink to the center of the new merged galaxy and form a binary system. We employ a semianalytic model of galaxy and quasar formation based on the hierarchical clustering scenario to estimate the amplitude of the expected stochastic gravitational wave background due to inspiraling SMBH binaries and bursts due to the SMBH binary coalescence events. We find that the characteristic strain amplitude of the background radiation is h(c)(f) similar to 10(-16)(f/1 muHz)(-2/3) for f less than or similar to 1 muHz just below the detection limit from measurements of the pulsar timing provided that SMBHs coalesce simultaneously when host galaxies merge. The main contribution to the total strain amplitude of the background radiation comes from SMBH coalescence events at 0 < z < 1. We also find that a future space-based gravitational wave interferometer such as the planned Laser Interferometer Space Antenna might detect intense gravitational wave bursts associated with coalescence of SMBH binaries with total mass M-tot < 10(7) M-circle dot at z greater than or similar to 2 at a rate similar to 1.0 yr(-1). Our model predicts that burst signals with a larger amplitude h(burst) similar to 10(-15) correspond to coalescence events of massive SMBH binary with total mass M-tot similar to 10(8) M-circle dot at low redshift (z less than or similar to 1) at a rate similar to 0.1 yr(-1), whereas those with a smaller amplitude (h(burst) similar to 10(-17)) correspond to coalescence events of less massive SMBH binaries with total mass M-tot similar to 10(6) M-circle dot at high redshift (z greater than or similar to 3).