Low-frequency gravitational radiation from coalescing massive black hole binaries in hierarchical cosmologies

We compute the expected low-frequency 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 dark matter halos and associated MBHs is followed via cosmological Monte Carlo realizations of the merger hierarchy from redshift z = 20 to the present in a \u39bCDM cosmology. MBHs get incorporated through halo mergers into larger and larger structures, sink to the center because of dynamical friction against the dark matter background, accrete cold material in the merger remnant, and form MBH binary systems. Stellar dynamical (three-body) 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. A simple scheme is applied in which the "loss cone" is constantly refilled and a constant stellar density core forms because of the ejection of stars by the shrinking binary. The integrated emission from inspiraling MBH binaries at all redshifts is computed in the quadrupole approximation and results in a gravitational wave background (GWB) with a well-defined shape that reflects the different mechanisms driving the late orbital evolution. The characteristic strain spectrum has the standard hc(f) f-2/3 behavior only in the range f = 10-9 to 10-6 Hz. At lower frequencies the orbital decay of MBH binaries is driven by the ejection of background stars ("gravitational slingshot"), and the strain amplitude increases with frequency, hc(f) f. In this range the GWB is dominated by 109-1010 M MBH pairs coalescing at 0 z 2. At higher frequencies, f > 10-6 Hz, the strain amplitude, as steep as hc(f) f-1.3, is shaped by the convolution of last stable circular orbit emission by lighter binaries (102-107 M) populating galaxy halos at all redshifts. We discuss the observability of inspiraling MBH binaries by a low-frequency gravitational wave experiment such as the planned Laser Interferometer Space Antenna (LISA). Over a 3 yr observing period LISA should resolve this GWB into discrete sources, detecting 60 (250) individual events above an S/N = 5 (S/N = 1) confidence level

The gravitational wave signal from massive black hole binaries and its contribution to the LISA data stream

Massive black hole binaries, with masses in the range 1E3-1E8 Msun, are expected to be the most powerful sources of gravitational radiation at mHz frequencies, and hence are among the primary targets for the planned Laser Interferometer Space Antenna (LISA). We extend and refine our previous analysis (Sesana et al. 2004), detailing the gravitational wave signal expected from a cosmological population of massive black hole binaries. As done in our previous paper, we follow the merger history of dark matter halos, the dynamics of the massive black holes they host, and their growth via gas accretion and binary coalescences in a LCDM cosmology. Stellar dynamical processes dominates the orbital evolution of black hole binaries at large separations, while gravitational wave emission takes over at small radii, causing the final coalescence of the pairs. We show that the GW signal from this population, in a 3 year LISA observation, will be resolved into approx 90 discrete events with S/N>5, among which approx. 35 will be observed above threshold until coalescence. These "merging events" involve relatively massive binaries, M=10E5 Msun, in the redshift range 2<z<6. The remaining approx. 55 events come from higher redshift, less massive binaries (M=5E3 Msun at z>6) and, although their S/N integrated over the duration of the observation can be substantial, the final coalescence phase is at too high frequency to be directly observable by space-based interferometers such as LISA. LISA will be able to detect a fraction approx. 90% of all the coalescences of massive black hole binaries occurring at z=5. The residual confusion noise from unresolved massive black hole binaries is expected to be at least an order of magnitude below the estimated stochastic noise.Comment: 9 pages, 9 figures, version accepted by Ap

Low-frequency gravitational radiation from coalescing massive black holes

We compute the expected low-frequency 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 dark matter halos and associated MBHs is followed via cosmological Monte Carlo realizations of the merger hierarchy from redshift z = 20 to the present in a ΛCDM cosmology. MBHs get incorporated through halo mergers into larger and larger structures, sink to the center because of dynamical friction against the dark matter background, accrete cold material in the merger remnant, and form MBH binary systems. Stellar dynamical (three-body) 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. A simple scheme is applied in which the "loss cone" is constantly refilled and a constant stellar density core forms because of the ejection of stars by the shrinking binary. The integrated emission from inspiraling MBH binaries at all redshifts is computed in the quadrupole approximation and results in a gravitational wave background (GWB) with a well-defined shape that reflects the different mechanisms driving the late orbital evolution. The characteristic strain spectrum has the standard hc(f) f-2/3 behavior only in the range f = 10-9 to 10-6 Hz. At lower frequencies the orbital decay of MBH binaries is driven by the ejection of background stars ("gravitational slingshot"), and the strain amplitude increases with frequency, hc(f) f. In this range the GWB is dominated by 109-1010 M MBH pairs coalescing at 0 z 2. At higher frequencies, f > 10-6 Hz, the strain amplitude, as steep as hc(f) f-1.3, is shaped by the convolution of last stable circular orbit emission by lighter binaries (102-107 M) populating galaxy halos at all redshifts. We discuss the observability of inspiraling MBH binaries by a low-frequency gravitational wave experiment such as the planned Laser Interferometer Space Antenna (LISA). Over a 3 yr observing period LISA should resolve this GWB into discrete sources, detecting 60 (250) individual events above an S/N = 5 (S/N = 1) confidence level