Low-frequency gravitational waves from supermassive black holes

Supermassive black holes are investigated as possible sources for low-frequency bursts of gravitational waves. The event rate for ‘known’ supermassive black holes at intermediate and high redshifts, inferred from the quasar luminosity function, is low, ∼ 0.1 yr–1 . A number density of gravitational wave sources, which is comparable to the number density of galaxies inferred from faint galaxy counts, is necessary to raise the event rate significantly above one per year. A space-based interferometer could therefore only see several events per year from supermassive black holes if an additional population of supermassive black holes existed and emitted gravitational waves efficiently. These might reside in the population of dwarf galaxies, or in a transient population of small dark-matter haloes that have, for the most part, merged into larger haloes hosting the galaxies seen today, as proposed in a hierarchical cosmogony. In the latter case, event rates could be of the order of 10–1000 per year, due to coalescing supermassive-biack-hole binaries formed during the merging process. Event rates could be as high as a few per second if the dark matter in galactic haloes consisted of supermassive black holes in the mass range 103 – 106 M⊙, and produced gravitational waves efficiently. The proposed space-based gravitational wave interferometer LISA/SAGITTARIUS should detect most gravitational wave events involving supermassive black holes above 104⊙, out to redshifts of z ∼ 100.