Intermediate-mass Black Holes on the Run from Young Star Clusters

Abstract

The existence of black holes (BHs) with masses in the range between stellar remnants and supermassive BHs has only recently become unambiguously established. GW190521, a gravitational wave signal detected by the LIGO/Virgo Collaboration, provides the first direct evidence for the existence of such intermediate-mass BHs (IMBHs). This event sparked and continues to fuel discussion on the possible formation channels for such massive BHs. As the detection revealed, IMBHs can form via binary mergers of BHs in the “upper mass gap” (≈40–120 M ⊙). Alternatively, IMBHs may form via the collapse of a very massive star formed through stellar collisions and mergers in dense star clusters. In this study, we explore the formation of IMBHs with masses between 120 and 500 M ⊙ in young, massive star clusters using state-of-the-art Cluster Monte Carlo models. We examine the evolution of IMBHs throughout their dynamical lifetimes, ending with their ejection from the parent cluster due to gravitational radiation recoil from BH mergers, or dynamical recoil kicks from few-body scattering encounters. We find that all of the IMBHs in our models are ejected from the host cluster within the first ∼500 Myr, indicating a low retention probability of IMBHs in this mass range for globular clusters today. We estimate the peak IMBH merger rate to be  ≈ 2 Gpc − 3 yr − 1 at redshift z ≈ 2.

Simulations predict intermediate-mass black hole formation in globular clusters

“…We now consider whether IMBHs could still be present in Milky Way GCs. Some previous studies ( 14 , 20 ) have found that IMBHs are ejected from their host clusters when they merge with another BH due to asymmetric gravitational wave emission (as discussed above). The recoil velocity from such a merger depends on the spin of both BHs and their masses.…”

“…In the center of GCs, their most dense region, stars and BHs can collide and merge, potentially producing IMBHs ( 11 , 18 , 19 ). Numerical simulations have predicted that mergers of binary BHs can form IMBHs up to ~500 M ⨀ ( 12 – 14 ). However, BH mergers emit gravitational waves.…”

“…However, BH mergers emit gravitational waves. If the masses or spins of the BHs differ from each other, then the emission is asymmetric, imparting a recoil kick velocity that can eject the resulting IMBH from its host cluster ( 14 , 20 ). This ejection mechanism prevents low-mass IMBHs from growing to more massive IMBHs through BH-IMBH mergers in GCs.…”

“…At the low-mass end of the IMBH range, the observation of gravitational wave event GW 190521 has indicated the formation of a 146 M ⨀ BH in a merger of two BHs with masses of 85 and 66 M ⨀ ( 10 ). Theoretical analyses have predicted that such merger events most often occur in star clusters, where the density of stars and BHs is high ( 11 – 14 ). At the high-mass end of the IMBH range, observations of active galactic nuclei in dwarf galaxies have indicated the presence of IMBHs with masses of 10 4 to 10 5 M ⨀ ( 15 ).…”