Possible LISA follow-on mission scientific objectives

A major objective that has been suggested for a follow-on mission to a Laser Interferometer Space Antenna (LISA)-type mission is to investigate more completely how intermediate mass black holes were formed and grew in the early universe, before they evolved into the much more massive black holes at the centers of many galaxies today. The actual design of such a follow-on mission will of course depend on what is observed by a LISA-type mission, such as the recently modified proposal for an evolved LISA mission, with the interferometer arm lengths between spacecraft reduced from 5 million to 1 million km. However, the sensitivity goals of a follow-on mission are likely to be influenced strongly by the desire to be able to see mergers of 10 M-circle dot black holes with roughly 3000 M-circle dot or larger intermediate mass black holes out to as large redshifts as possible. Approximate calculations of the expected signal-to-noise have been made for a possible LISA follow-on mission that was suggested about eight years ago (Bender and Begelman 2005 Trends in Space Science and Cosmic Vision 2020 (Noordwijk: ESA Publications Division) pp 33-38), and was called the Advanced Laser Interferometer Antenna. Based on the calculations, it appears that detections out to a redshift of 10 would be possible for 10 M-circle dot black holes spiraling into perhaps 5000 M-circle dot or larger intermediate mass black holes if the extragalactic gravitational wave background due to close white dwarf binaries is in the currently estimated range.