Simulations predict intermediate-mass black hole formation in globular clusters

The formation process of intermediate-mass black holes (IMBHs), defined as those between 100 and 10⁵ solar masses (M_⊙), is debated. One potential origin is the growth of less-massive black holes merging with stars and compact objects within globular clusters (GCs). However, previous simulations have indicated that this process only produces IMBHs under 500 M_⊙ before gravitational wave recoil ejects them from the GC. We performed star-by-star simulations of GC formation, finding that high-density star formation in a GC's parent giant molecular cloud can produce sufficient mergers of massive stars to overcome that mass threshold. We conclude that GCs can form with IMBHs more than 10³ M_⊙, which is sufficiently massive to be retained within the GC even with the expected gravitational wave recoil.