GRACE-BH · Gravitational waves from crowded environments: simulating intermediate-mass black hole formation and evolution with supercomputers.

Gravitational waves from crowded environments: simulating intermediate-mass black hole formation and evolution with supercomputers.

Intermediate-mass black holes represent an elusive class of objects that are supposed to link stellar-mass black holes, formed from the death of massive stars, and supermassive black holes, which are routinely found in the centre of galaxies. While stellar black holes have masses from a few to tens of times the mass of the Sun (solar mass), supermassive black holes can attain masses from millions to tens of billions of solar masses. Intermediate-mass black holes, referred to as IMBHs in what follows, are believed to sit in the middle, with masses from a hundred to a few hundred thousand solar masses. Only a few IMBH candidates, with masses around 50,000 solar masses, have been observed in the centre of dwarf galaxies, and only a dozen controversial observations found them in globular clusters. In 2021, the LIGO-Virgo-Kagra collaboration discovered the first IMBH with a mass of 150 solar masses, formed from the merger of two smaller black holes. The lack of an observational smoking gun in the broad 1,000-100,000 solar mass range, however, makes it hard to assess whether IMBHs constitute a class of black holes or, rather, they are representatives of the populations of high-mass stellar black holes and low-mass supermassive black holes. Proving that the former hypothesis is true would have implications for the formation of supermassive black holes and the evolution of dense stellar environments. Understanding whether IMBHs link stellar and supermassive black holes requires finding a set of processes and environments capable of supporting the formation of IMBHs through the whole 100-100,000 solar mass range. The GRACE-BH proposes to tackle this astrophysical challenge by addressing the following question: What are the best conditions under which an intermediate-mass black hole forms in a massive stellar system? Utilising unprecedented, state-of-the-art numerical simulations to model the evolution of dense star clusters, the GRACE-BH project explores whether, and how, IMBHs can form and possibly grow in such extreme environments. In brief, the GRACE-BH main objectives are: 1) quantify the impact of mass-segregation on the formation of an IMBH 2) quantify the IMBH seed survival probability 3) quantify possible gravitational-wave signatures associated with the IMBH growth process 4) determine the most favourable environments to nurture IMBHs By achieving these objectives, the GRACE-BH project provided clear evidence of the existence of particular star cluster configurations ideal for the seeding and growth of IMBHs.

Data: CORDIS, © European Union