 Award ID(s):
 2110507
 NSFPAR ID:
 10427934
 Date Published:
 Journal Name:
 The Astrophysical Journal Letters
 Volume:
 935
 Issue:
 1
 ISSN:
 20418205
 Page Range / eLocation ID:
 L20
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
More Like this

Abstract Ultralight bosons are a proposed solution to outstanding problems in cosmology and particle physics: they provide a darkmatter candidate while potentially explaining the strong chargeparity problem. If they exist, ultralight bosons can interact with black holes through the superradiant instability. In this work we explore the consequences of this instability on the evolution of hierarchical black holes within dense stellar clusters. By reducing the spin of individual black holes, superradiance reduces the recoil velocity of merging binary black holes, which, in turn, increases the retention fraction of hierarchical merger remnants. We show that the existence of ultralight bosons with mass 2 × 10 −14 ≲ μ /eV ≲ 2 × 10 −13 would lead to an increased rate of hierarchical black hole mergers in nuclear star clusters. An ultralight boson in this energy range would result in up to ≈60% more presentday nuclear star clusters supporting hierarchical growth. The presence of an ultralight boson can also double the rate of intermediatemass black hole mergers to ≈0.08 Gpc −3 yr −1 in the local universe. These results imply that a select range of ultralight boson masses can have farreaching consequences for the population of black holes in dense stellar environments. Future studies into black hole cluster populations and the spin distribution of hierarchically formed black holes will test this scenario.more » « less

Abstract It has been proposed that some black holes (BHs) in binary black hole (BBH) systems are born from “hierarchical mergers” (HMs), i.e., earlier mergers of smaller BHs. These HM products have spin magnitudes χ ∼ 0.7, and, if they are dynamically assembled into BBH systems, their spin orientations will sometimes be antialigned with the binary orbital angular momentum. In fact, as Baibhav et al. showed, ∼16% of BBH systems that include HM products will have an effective inspiral spin parameter, χ eff < −0.3. Nevertheless, the LIGO–Virgo–KAGRA (LVK) gravitationalwave (GW) detectors have yet to observe a BBH system with χ eff ≲ −0.2, leading to upper limits on the fraction of HM products in the population. We fit the astrophysical mass and spin distribution of BBH systems and measure the fraction of BBH systems with χ eff < −0.3, which implies an upper limit on the HM fraction. We find that fewer than 26% of systems in the underlying BBH population include HM products (90% credibility). Even among BBH systems with primary masses m 1 = 60 M ⊙ , the HM fraction is less than 69%, which may constrain the location of the pairinstability mass gap. With 300 GW events (to be expected in the LVK’s next observing run), if we fail to observe a BBH with χ eff < −0.3, we can conclude that the HM fraction is smaller than 2.5 − 2.2 + 9.1 % .more » « less

ABSTRACT Merging black holes (BHs) are expected to produce remnants with large dimensionless spin parameters (aspin ∼ 0.7). However, gravitational wave (GW) observations with LIGO–Virgo–Kagra (LVK) suggest that merging BHs are consistent with modestly positive but not high spin (aspin ∼ 0.2), causing tension with models suggesting that highmass mergers are produced by hierarchical merger channels. Some BHs also show evidence for strong inplane spin components. Here, we point out that spindown of BHs due to eccentric prograde postmerger orbits within the gas of an active galactic nucleus (AGN) disc can yield BHs with masses in the upper mass gap, but only modestly positive aspin, and thus observations of BHs with low spin do not rule out hierarchical models. We also point out that the fraction of binary black hole (BBH) mergers with significant inplane spin components is a strong test of interactions between disc BBHs and nuclear spheroid orbiters. Spin magnitude and spin tilt constraints from LVK observations of BBHs are an excellent test of dynamics of BHs in AGN discs, disc properties, and the nuclear clusters interacting with AGNs.

All ten LIGO/Virgo binary black hole (BHBH) coalescences reported following the O1/O2 runs have nearzero effective spins. There are only three potential explanations for this. If the BH spin magnitudes are large, then: (i) either both BH spin vectors must be nearly in the orbital plane or (ii) the spin angular momenta of the BHs must be oppositely directed and similar in magnitude. Then there is also the possibility that (iii) the BH spin magnitudes are small. We consider the third hypothesis within the framework of the classical isolated binary evolution scenario of the BHBH merger formation. We test three models of angular momentum transport in massive stars: a mildly efficient transport by meridional currents (as employed in the Geneva code), an efficient transport by the TaylerSpruit magnetic dynamo (as implemented in the MESA code), and a veryefficient transport (as proposed by Fuller et al.) to calculate natal BH spins. We allow for binary evolution to increase the BH spins through accretion and account for the potential spinup of stars through tidal interactions. Additionally, we update the calculations of the stellarorigin BH masses, including revisions to the history of star formation and to the chemical evolution across cosmic time. We find that we can simultaneously match the observed BHBH merger rate density and BH masses and BHBH effective spins. Models with efficient angular momentum transport are favored. The updated stellarmass weighted gasphase metallicity evolution now used in our models appears to be key for obtaining an improved reproduction of the LIGO/Virgo merger rate estimate. Mass losses during the pairinstability pulsation supernova phase are likely to be overestimated if the merger GW170729 hosts a BH more massive than 50 M ⊙ . We also estimate rates of black holeneutron star (BHNS) mergers from recent LIGO/Virgo observations. If, in fact. angular momentum transport in massive stars is efficient, then any (electromagnetic or gravitational wave) observation of a rapidly spinning BH would indicate either a very effective tidal spin up of the progenitor star (homogeneous evolution, highmass Xray binary formation through case A mass transfer, or a spin up of a WolfRayet star in a close binary by a close companion), significant mass accretion by the hole, or a BH formation through the merger of two or more BHs (in a dense stellar cluster).more » « less

Abstract We describe the public release of the Cluster Monte Carlo (
CMC ) code, a parallel, starbystarN body code for modeling dense star clusters.CMC treats collisional stellar dynamics using Hénon’s method, where the cumulative effect of many twobody encounters is statistically reproduced as a single effective encounter between nearestneighbor particles on a relaxation timescale. The starbystar approach allows for the inclusion of additional physics, including strong gravitational three and fourbody encounters, twobody tidal and gravitationalwave captures, mass loss in arbitrary galactic tidal fields, and stellar evolution for both single and binary stars. The public release ofCMC is pinned directly to theCOSMIC population synthesis code, allowing dynamical star cluster simulations and population synthesis studies to be performed using identical assumptions about the stellar physics and initial conditions. As a demonstration, we present two examples of star cluster modeling: first, we perform the largest (N = 10^{8}) starbystarN body simulation of a Plummer sphere evolving to core collapse, reproducing the expected selfsimilar density profile over more than 15 orders of magnitude; second, we generate realistic models for typical globular clusters, and we show that their dynamical evolution can produce significant numbers of black hole mergers with masses greater than those produced from isolated binary evolution (such as GW190521, a recently reported merger with component masses in the pulsational pairinstability mass gap).