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For the purposes of LISA, perhaps the most interesting astrophysics is that of massive black holes (MBHs) in the centers of galaxies. These are 10^6 to 10^8 solar masses in size. Such black holes are now believed to be common in the nuclei of normal galaxies, and there is even compelling evidence that our own galaxy shelters a black hole of around three million solar masses in its center. It is not yet clear exactly how MBHs are formed, but it is plausible that they could have been created by the coalescence of smaller "seed" black holes or from the collapse of dense gas clouds, super-massive stars, or relativistic star clusters; or by coalescing from galaxy mergers. Two current models that would lead to gravitational wave signals easily detectable by LISA are the non-axisymmetric collapse of massive gas clouds and the runaway merging of compact objects in dense star clusters. On the other hand, if these black holes are formed by slow accretion onto a small seed, no appreciable gravitational wave signal will be present. LISA might be able to see the birth of massive black holes, resolving these and other interesting astrophysical questions.
MBHs play also a major role in at least two of the "guaranteed" sources for LISA (sources which, if unseen by LISA, would cause us to re-evaluate the very existence of gravitational waves). The first interesting source involving massive black holes is their merger during a galactic merger. Such an event will be visible to LISA from anywhere in the universe with signal-to-noise ratios of up to thousands. The only outcome of such an event allowed by general relativity is the production of a new, larger black hole with a perturbed event horizon. Gravitational waves from perturbed horizons are distinctive and well understood, so either black hole perturbation theory can be confirmed with high precision or general relativity will be called into question.
The gravitational waves from the actual merger are not well understood, but numerical simulations of these events are being actively pursued by groups from around the world. Detected mergers can be compared to numerical results to provide insight and guide development of simulation software (see the Source Modeling section). Astrophysically, such events will confirm (or prove false) our current belief that massive black holes are found in normal galactic nuclei, and event rates will constrain a combination of merger rates of galaxies at redshift of Z=0 to Z=10, formation models for massive black holes in galactic nuclei, and the rate of dynamical friction in the loss-cone regime. Observation of interacting and merging galaxies taken by the Hubble and SDSS telescopes seems to justify the possibility that the rate of these events could be several per year.
The second possible source for LISA is the inspiral of compact objects of stellar to moderate mass (1 - 10^4 solar masses) into massive black holes. Signals from such sources can be modeled using perturbation theory in general relativity, and therefore will (in principle) be available for data analysis. Encoded in these waveforms is a detailed map of the space-time structure in the vicinity of the massive black hole that will give us by far the most sensitive probe of general relativity in strong field regimes to date. Astrophysical inputs/outputs from the event rates for these signals include the initial mass function and mass segregation in the central parsec of galactic nuclei and the cross-section for scattering into the loss cone. In active galactic nuclei, information about accretion disk drag may also be available.
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