LISA will have the capacity to detect the mergers of massive black holes out to high-z (z > 10) being particularly sensitive to black holes in the range 104 to 107 M☉. This mass range encompasses the so-called “Intermediate Mass Black Holes” range. However, these black holes, due to their intrinsic size and luminosity, are particularly challenging to detect. Moreover, their number densities are completely unknown.
The Maynooth Black Holes group use state-of-the art computational and theoretical models to probe the formation and growth of astrophysical black holes in the early Universe. If, as predicted, black holes grow from seeds (see Figure 1) to eventually form the super-massive black holes we observe today then they must pass through the intermediate stage (see Figure 2). Therefore to know the abundances of the Intermediate mass black holes we need to know the abundances of the seeds. Therefore the question of the abundances of these massive black hole seeds then becomes centre stage. If we know the abundance of the seeds, then we can also estimate how many mergers of intermediate mass black holes should happen in our Universe. This is what LISA should see!
As the time to LISA’s launch decreases, the fidelity of our numerical models increases allowing us to make more and more accurate predictions for the number densities of massive black holes in our Universe. To do so our computer models need to model the formation of virtual universes encompassing galaxy formation, star formation and black hole formation and the multitude of physical processes that accompany all of these observables. The recently launched JWST space telescope has shown us that massive black holes exist and are growing in the early Universe. These (EM) observations are already helping to constrain our models. Of course ultimately what we need is LISA’s observations to constrain our models even further!