Testing Models of Supermassive Black Hole Seed Formation through Gravity Waves

Abstract

We study the gravitational wave background produced from the formation and assembly of supermassive black holes within the cosmological paradigm of hierarchical structure formation. In particular, we focus on a supermassive black hole formation scenario in which the present-day population of supermassive black holes is built from high-mass seed black holes and we compute the concomitant spectrum of gravitational radiation produced by mergers of the seed black holes. We find that this scenario predicts a large, gravitational wave background that should be resolved into individual sources with space interferometers such as the proposed Laser Interferometric Space Antenna (LISA). The number of inspiral, merger and ringdown events above a signal to noise ratio of 5 that result from massive black hole seeds is of order 10^3. This prediction is robust and insensitive to several of the details of the model. We conclude that an interferometer such as LISA will be able to effectively rule out or confirm a class of models where supermassive black holes grow from high-mass seed black holes and may be able to place strong limits on the role of mergers as a channel for supermassive black hole growth. Supermassive black hole seeds likely form in the earliest proto-galactic structures at high redshift and the masses of supermassive black holes are known to be strongly correlated with the potentials of the spheroids in which they reside, therefore these results imply that space interferometers can be used as a powerful probe of the physics of galaxy formation and proto-galaxy formation at very high redshift.