Low-frequency gravitational waves from supermassive black holes

Supermassive black holes are investigated as possible sources for low-frequency bursts of gravity waves. The event rate for `known' supermassive black holes at intermediate and high redshifts, inferred from the quasar luminosity function, is low $\sim 0.1 \yr^{-1}$. 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 mostly merged into larger haloes hosting the galaxies seen today. The proposed space-based gravitational-wave interferometer LISA/SAGITTARIUS should detect most gravitational-wave events involving supermassive black holes above $10^{4} \Msol$ out to redshifts of $z \sim 100$.