Studying massive black hole mergers are a primary science objective of LISA.
If the present galaxies were formed from smaller structures which have merged, there may have been many opportunities for coalescence of massive black holes from the smaller structures. If a great many galaxies containing massive black holes undergo mergers, and the massive black holes fall to the center by dynamical friction in a Hubble time, then the resulting massive black hole binaries would coalesce and generate large amounts of gravitational radiation. The event rate could be several per year.
A coalescence of two 106 solar mass black holes at z=1 would produce a signal-to-noise ratio of several thousand after 1 year integration. These signal strengths may be sufficient to direct electromagnetic observations towards a specific angular window on the sky in the last few days before coalescence.
These studies will answer basic questions in physics and astrophysics
1. Allow precision tests of dynamical non-linear gravity
2. What is the role of massive black holes in galaxy evolution?
3. What fraction of galactic merger events result in a massive black hole merger?
4. When were the earliest massive black hole mergers?
Science measurements will provide a comparison of the inspiral waveform with predictions of numerical general relativity. LISA will be able to measure the rate of massive black hole mergers with z, the mass spectrum of massive black holes in merger events and the spin of massive black holes.
From LISA System and Technology Study Report (2000) (Fig 1.4)
ESA-SCI(2000)11