The possible pathways leading to the formation of a supermassive black hole have been laid out in a seminal paper by Martin Rees (Rees 1984), and include the direct collapse of a massive gas cloud into one single object, the formation of star clusters with subsequent run-away collapse and stellar mergers, and including possible intermediate stages such as a supermassive star or a cluster of neutron stars and black holes. An updated version of these scenarios within our modern cosmological framework has been given by Regan & Haehnelt (2009), see also Fig. 1.
As the formation of massive seed black holes requires a substantial gas reservoir, their formation is generally considered to take place in so-called atomic cooling halos with virial temperatures of at least 104 K, implying that cooling via atomic hydrogen lines is in principle possible. The thermal evolution in these halos strongly depends on their history, such as previous mergers and metal enrichment, or the presence of an external radiative background. If cooling is efficient, due to the presence of metals or molecular hydrogen, then fragmentation is expected to occur, therefore likely favoring the formation of a massive star cluster (e.g. Omukai et al. 2008). If cooling is suppressed, as in a primordial gas exposed to a strong photodissociating radiation background, fragmentation is likely to be very inefficient, leading to the formation of potentially very massive objects (e.g. Schleicher et al. 2010, Latif & Schleicher 2015). It is therefore essential to assess if these pathways are possible, under what precise conditions they occur, and what the outcome plausibly is. In addition, as previous studies already established that the gravitational wave signatures from direct collapse black hole formation as well as the coalescence of the resulting seed black holes could be detected with space-born gravitational wave observatories such as LISA and DECIGO (Koushiappas et al. 2006; Reisswig et al. 2013; Pacucci et al. 2015), we will aim here at predicting event rates on the basis of realistic models.
We will explore the hypothesis that massive seed black holes have formed in dense stellar clusters, which naturally formed in the early Universe in the presence of H 2 cooling or metal enrichment. We plan to assess this possibility through dedicated simulations exploring both the evolution of clusters from realistic initial conditions, taking into account the gas phase processes that are likely going to affect the evolution of the clusters. In addition, we will explore the growth of black hole seeds after their formation, with the goal of studying the accretion physics in the presence of strong self-gravitational instabilities, with the goal to compare to accretion events in highly obscured sources as observed with NuSTAR, as well as the modeling of accretion in very nearby galaxies, where it can be observed with ALMA on scales less than 100 pc.
By combining our black hole formation models with a cosmological framework, we will derive gravitational wave event rates both on the bases of the formation event itself due to the collapse of a supermassive star (see Reisswig et al. 2013), as well as events produced by the mergers of the massive seed black holes (Koushiappas et al. 2006). Such predictions will be pursued both for conservative and more optimistic formation scenarios, and compared to the expected sensitivities of LISA and DECIGO. With anticipated launch dates in 2034 and 2027, this work is clearly long-term, but also the preparatory work on LIGO has been pursued over a long timescale. For this work, we plan to strongly interact with the European community preparing for LISA. In particular, our international collaborator Monica Colpi has great expertises in calculating gravitational wave signatures and event rates, and Lucio Mayer is a member of the International LISA Consortium and representative of the European network of scientists included in the new COST Action Gravitational Waves, Black Holes and Fundamental Physics. Our results on collisions in dense star clusters will further be employed by our international collaborator Raffaella Schneider, to make predictions for the Advanced LIGO (see also Schneider et al. 2017).