Michela Mapelli
Traditionally, IMBHs are expected to form through hierarchical mergers of stellar-origin black holes and runaway stellar collisions in dense stellar environments, possibly assisted by gas accretion. However, numerical models rely on the sticky-particle technique, which might spuriously boost the formation efficiency by orders of magnitude. In my talk, I will present new hydrodynamical simulations of star-star collisions showing that this mechanism struggles to efficiently produce IMBHs, especially when the colliding stars are in the post-main-sequence phase. Specifically, both mass ejection in the collisions and radiation-driven winds significantly increase the mass loss and prevent IMBH formation. In contrast, our models show that hierarchical binary black hole mergers can lead to IMBH formation, but only in high-redshift nuclear star clusters. The formation efficiency of IMBHs critically depends on the available gas reservoir and on the ability of nuclear star clusters to accrete other clusters. I will also discuss how current and upcoming gravitational-wave data can be used to constrain the efficiency of IMBH formation.
Amphithéâtre Henri Mineur, Institut d'Astrophysique de Paris