We compute the expected gravitational wave signal from coalescing massive
black hole (MBH) binaries at the center of galaxies in a hierarchical structure
formation scenario in which seed holes of intermediate mass form far up in the
dark halo merger tree. The merger history of DM halos and MBHs is followed from
z=20 to the present in a LCDM cosmology. MBHs get incorporated through halo
mergers into larger and larger structures, sink to the center owing to
dynamical friction against the DM background, accrete cold material in the
merger remnant, and form MBH binary systems. Stellar dynamical interactions
cause the hardening of the binary at large separations, while gravitational
wave emission takes over at small radii and leads to the final coalescence of
the pair. The integrated emission from inspiraling MBH binaries results in a
gravitational wave background (GWB). The characteristic strain spectrum has the
standard h_c(f)\propto f^{-2/3} behavior only in the range 1E-9<f<1E-6 Hz. At
lower frequencies the orbital decay of MBH binaries is driven by the ejection
of background stars, and h_c(f) \propto f. At higher frequencies, f>1E-6 Hz,
the strain amplitude is shaped by the convolution of last stable circular orbit
emission. We discuss the observability of inspiraling MBH binaries by the
planned LISA. Over a 3-year observing period LISA should resolve this GWB into
discrete sources, detecting ~60 (~250) individual events above a S/N=5 (S/N=1)
confidence level. (Abridged)Comment: 11 pages, 8 figues. Revised version accepted to be published in ApJ
Discussion on number counts corrected and expande
