To achieve the goals set by the GRACE-BH project, I initially invested a significant amount of time to implement, upgrade, and test, the NBODY6++GPU code, a direct N-body code that permits the simulation of the evolution of star clusters simultaneously taking into account both stellar evolution and dynamics.
Using the upgraded code and exploiting the JUWELS Booster supercomputer, the fastest supercomputer in Europe and fifth in the world in 2022, we gathered an unprecedented sample of 19 N-body models of young massive star clusters, referred to as DRAGON-II simulations.
In DRAGON-II models, I identified three different processes that can lead to the formation of an IMBH: 1) stellar collisions, generally involving stars with a mass larger than 100 solar masses, 2) accretion of material from a stellar relic onto a black hole, 3) black hole - black hole mergers.
These processes are not mutually exclusive. For example, an IMBH seed can initially form via stellar collisions and the resulting IMBH can later grow by feasting on passing by stars.
For the first time, we found that star clusters with a density above 300,000 solar masses per cubic parsec form IMBHs through stellar collisions on very short timescales (a few million years), whilst sparser clusters form IMBHs via black hole mergers and stellar accretion episodes on hundreds of million years.
In clusters with masses above 1-10 million solar masses and sizes ~1 parsec, a cascade of repeated black hole mergers can onset and rapidly build up IMBHs as massive as 10,000 solar masses, regardless of the IMBH initial mass.
We found a substantial population of compact binary mergers and used them to infer the cosmic merger rate of black holes from star clusters. Exotic mergers involve black holes in the so-called upper mass gap, IMBHs, white dwarfs and neutron stars. We place constraints on the detectability of such sources with current and future gravitational wave detectors, possibly jointly with electromagnetic facilities. For the first time, we predict the detection rate of pair-instability supernovae from star clusters, an elusive type of cosmic explosion expected from the death of stars more massive than 100 solar masses.
To complement the limited sample of DRAGON-II models, I also used the semi-analytic code B-POP to study the growth of IMBHs via multiple stellar black hole mergers in a statistical way. I found two possible regimes leading to IMBH formation. The first in clusters with masses in the range 1-10 million solar masses per cubic parsec and pc-scale sizes, where densities are sufficiently large to trigger an avalanche of black hole mergers that rapidly builds-up an IMBH with final masses above 10,000 solar masses. The second in clusters where stellar processes (e.g. stellar collisions) can form an IMBH seed that further grows via repeated mergers with smaller black holes.
To communicate GRACE-BH results to the broad public, I was also involved in several outreach activities.
