FIGURE 2.2 The source 3C 75, shown here in X-rays (blue) and radio waves (pink), is a rare example of two galaxies caught in the act of merging. Not only do their stars merge, but their central black holes—each producing a pair of jets containing gas moving outward at a speed close to that of light—also will do likewise in perhaps a few hundred million years. Many similar mergers involving smaller black holes in the nuclei of younger galaxies are thought to have taken place. When black holes coalesce, they create intense bursts of gravitational radiation. SOURCE: X-ray—NASA/CXC/AIfA/D. Hudson and T. Reiprich et al. Radio—NRAO/VLA/NRL.
signals will require deploying a space-based observatory with detectors separated by millions of kilometers to achieve the required sensitivity. Detection of these mergers would provide direct measurements of the masses and spins of supermassive black holes and the geometry of the universe on its largest scales. Powerful tests of our understanding of how black holes and galaxies form and evolve will be possible. We are on the verge of a new era of discovery in gravitational wave astronomy.