Black hole mergers in compact star clusters and massive black hole formation beyond the mass gap

Rizzuto, Francesco Paolo; Naab, Thorsten; Spurzem, Rainer; Sedda, Manuel Arca; Giersz, Mirek; Ostriker, Jeremiah P.; Banerjee, Sambaran

doi:10.1093/mnras/stac231

Cited by 37 publications

(17 citation statements)

References 72 publications

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“…This will enable self-consistent treatment of the evolution of black hole masses over time in CMC through the effects of TDEs and collisions. This may have important implications for the overall black hole mass spectrum in star clusters and the mass spectrum of binary black hole mergers that are now detectable as gravitational wave sources (Lopez & Batta 2019;Rizzuto et al 2022).…”

Section: Future Workmentioning

confidence: 99%

“…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).…”

Section: Introductionmentioning

confidence: 99%

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

Kremer

Lombardi

et al. 2022

ApJ

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Recent analyses have shown that close encounters between stars and stellar black holes occur frequently in dense star clusters. Depending upon the distance at closest approach, these interactions can lead to dissipating encounters such as tidal captures and disruptions, or direct physical collisions, all of which may be accompanied by bright electromagnetic transients. In this study, we perform a wide range of hydrodynamic simulations of close encounters between black holes and main-sequence stars that collectively cover the parameter space of interest, and we identify and classify the various possible outcomes. In the case of nearly head-on collisions, the star is completely disrupted with roughly half of the stellar material becoming bound to the black hole. For more distant encounters near the classical tidal-disruption radius, the star is only partially disrupted on the first pericenter passage. Depending upon the interaction details, the partially disrupted stellar remnant may be tidally captured by the black hole or become unbound (in some cases, receiving a sufficiently large impulsive kick from asymmetric mass loss to be ejected from its host cluster). In the former case, the star will undergo additional pericenter passages before ultimately being disrupted fully. Based on the properties of the material bound to the black hole at the end of our simulations (in particular, the total bound mass and angular momentum), we comment upon the expected accretion process and associated electromagnetic signatures that are likely to result.

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Section: Future Workmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Kremer

Lombardi

et al. 2022

ApJ

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“…Heuristically, if a collision between a BH and a star results in an accretion disk, the disk's viscous timescale may be as short as days. The resultant luminosity can unbind most of the captured material, though details such as the amount accreted and peak luminosity remain uncertain (e.g., Yuan et al 2012;Jiang et al 2014; see also the discussion in Stone et al 2017;Rizzuto et al 2022, andKremer et al 2022). The question becomes whether or not a BH can still accumulate significant amounts of mass over many collisions even if it accretes very little in a single one.…”

Section: Uncertainties In Accretionmentioning

confidence: 99%

“…However, as discussed in Section 2.2, direct BH-star collisions are much more frequent than BH-BH collisions in GNs, making the former a promising channel for BH growth. In an N-body study of young star clusters, Rizzuto et al (2022) find that BH-star collisions are the main contributor to the formation of BHs in the mass gap and IMBHs. In a similar vein, Stone et al (2017) demonstrate that massive BHs can form from repeated tidal encounters between stars and BHs.…”

Section: Introductionmentioning

confidence: 99%

The Formation of Intermediate-mass Black Holes in Galactic Nuclei

Rose

Naoz

Sari

et al. 2022

ApJL

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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.

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“…In this study, a young massive cluster (hereafter YMC) origin of PSN-gap BBH mergers is investigated. Dynamical interactions among stellar-remnant BHs inside dense star clusters is an intriguing scenario for generating PSN-gap BBH mergers as well as other types of massive BBH mergers (Perna et al 2019;Arca Sedda et al 2020;Baibhav et al 2020;Banerjee 2021a;Rizzuto et al 2021Rizzuto et al , 2022Gerosa & Fishbach 2021;Mapelli et al 2021). This is because the scenario naturally allows for the prolonged post-processing of BHs and BBHs that formed as a result of massive binary evolution.…”

Section: Introductionmentioning

confidence: 99%

Binary black hole mergers from young massive clusters in the pair-instability supernova mass gap

Banerjee¹

2022

A&A

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Context. The recent discovery of the binary black hole (BBH) merger event GW190521, between two black holes (BHs) of ≈100 Msamp, in addition to other massive BBH merger events involving BHs within the pair-instability supernova (PSN) mass gap have sparked widespread debate on the origin of such extreme gravitational-wave (GW) events. GW190521 simultaneously triggers two critical questions: how BHs can appear within the ‘forbidden’ PSN gap and, if they do, how they get to participate in general-relativistic (GR) mergers. Aims. In this study, I investigate whether dynamical interactions in young massive clusters (YMCs) serve as a viable scenario for assembling PSN-gap BBH mergers. Methods. To that end, I explore a grid of 40 new evolutionary models of a representative YMC of initial mass and size Mcl = 7.5 × 104 Msamp (N ≈ 1.28 × 105) and rh = 2 pc, respectively. The model grid ranges over metallicity 0.0002 ≤ Z ≤ 0.02 and comprises initial cluster configurations of King central concentration parameters W0 = 7 and 9. In each model, all BH progenitor stars are initially in primordial binaries following observationally motivated distributions. All cluster models are evolved with the direct, relativistic N-body code NBODY7, incorporating up-to-date remnant formation, BH natal spin, and GR merger recoil schemes. Results. Binary black hole mergers from these model cluster computations agree well with the masses and effective spin parameters, χeff, of the events from the latest gravitational-wave transient catalogue (GWTC). In particular, GW190521-like, that is to say ≈200 Msamp, low χeff events are produced via a dynamical merger among BHs derived from star-star merger products. GW190403_051519-like, that is PSN-gap, highly asymmetric, high χeff events result from mergers involving BHs that are spun up via matter accretion or a binary interaction. The resulting present-day, differential intrinsic merger rate density, within the PSN gap, accommodates that from GWTC well. Conclusions. This study demonstrates that, subject to model uncertainties, the tandem of massive binary evolution and dynamical interactions in ≲100 Myr-old, low metallicity YMCs in the Universe can plausibly produce GR mergers involving PSN-gap BHs and in rates consistent with that from up-to-date GW observations. Such clusters can produce extreme events similar to GW190521 and GW190403_051519. The upper limit of the models’ GW190521-type event rate is within the corresponding LIGO-Virgo-KAGRA (LVK)-estimated rate limits, although the typical model rate lies below LVK’s lower limit. The present YMC models yield a merger rate density of 0−3.8 × 10−2 yr−1 Gpc−3 for GW190521-type events. They produce GW190403_051519-like events at a rate within 0−1.6 × 10−1 yr−1 Gpc−3 and their total BBH-merger yield within the PSN gap is 0−8.4 × 10−1 yr−1 Gpc−3.

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Black hole mergers in compact star clusters and massive black hole formation beyond the mass gap

Cited by 37 publications

References 72 publications

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

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

The Formation of Intermediate-mass Black Holes in Galactic Nuclei

Binary black hole mergers from young massive clusters in the pair-instability supernova mass gap

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