The Formation of Intermediate-mass Black Holes in Galactic Nuclei

Abstract

Most stellar evolution models predict that black holes (BHs) should not exist above approximately 50–70 M ⊙, the lower limit of the pair-instability mass gap. However, recent LIGO/Virgo detections indicate the existence of BHs with masses at and above this threshold. We suggest that massive BHs, including intermediate-mass BHs (IMBHs), can form in galactic nuclei through collisions between stellar-mass BHs and the surrounding main-sequence stars. Considering dynamical processes such as collisions, mass segregation, and relaxation, we find that this channel can be quite efficient, forming IMBHs as massive as 104 M ⊙. This upper limit assumes that (1) the BHs accrete a substantial fraction of the stellar mass captured during each collision and (2) that the rate at which new stars are introduced into the region near the SMBH is high enough to offset depletion by stellar disruptions and star–star collisions. We discuss deviations from these key assumptions in the text. Our results suggest that BHs in the pair-instability mass gap and IMBHs may be ubiquitous in galactic centers. This formation channel has implications for observations. Collisions between stars and BHs can produce electromagnetic signatures, for example, from X-ray binaries and tidal disruption events. Additionally, formed through this channel, both BHs in the mass gap and IMBHs can merge with the SMBHs at the center of a galactic nucleus through gravitational waves. These gravitational-wave events are extreme- and intermediate-mass ratio inspirals.

Hydrodynamics of Collisions and Close Encounters between Stellar Black Holes and Main-sequence Stars

“…Aside from their intrinsic interest as electromagnetic transient sources, close encounters between black holes and stars may have an important effect on the overall demographics of black holes in clusters. If a significant fraction of the disrupted stellar material is accreted by the black hole, these TDEs may lead to significant black hole mass growth, potentially enabling formation of black holes within or above the pair-instability mass gap (e.g., Giersz et al 2015;Arca-Sedda et al 2021;Rizzuto et al 2022;Rose et al 2022). Furthermore, if a star is disrupted by a binary black hole, the disrupted material may alter the spin magnitude and orientation of the binary black hole components (e.g., Lopez & Batta 2019).…”

A Case for a Binary Black Hole System Revealed via Quasi-Periodic Outflows

“…These black holes can inspiral towards the central SMBH after their cluster gets disrupted by tides (37,39), and it is predicted to result in SMBH-IMBH merger rates of 10 −4 -10 −3 Gpc −3 yr −1 in the local Universe (40). IMBHs with masses ∼10 3−4 M ⊙ can also be retained in NSCs as a result of repeated stellar mass black hole-stellar-mass black hole or stellar-mass black hole-star mergers (41,42).…”

“…• a series of mergers of stellar black holes (41) or stellar black holes with other stars in the NSC (42).…”

Merger Rates of Intermediate-mass Black Hole Binaries in Nuclear Star Clusters

“…IMBHs with masses in between these two extremes can be produced at any redshift through repeated mergers either of massive mainsequence stars, later collapsing to form a BH (Portegies Zwart & McMillan 2002;Gürkan et al 2004;Freitag et al 2006;Pan et al 2012;Giersz et al 2015;Tagawa et al 2020;Das et al 2021;Di Carlo et al 2021), or of stellar-mass BHs (Miller & Hamilton 2002;O'Leary et al 2006;Antonini & Rasio 2016;Antonini et al 2019;González et al 2021;Mapelli et al 2021;Weatherford et al 2021;Fragione et al 2022). Other possibilities include the fragmentation of active galactic nucleus (AGN) disks (McKernan et al 2012(McKernan et al , 2014, super-Eddington accretion onto stellar BHs embedded in AGN disks (e.g., Kocsis et al 2011), and repeated mergers of BHs with stars in galactic nuclei (e.g., Stone et al 2017;Rose et al 2022).…”