We explore the masses, merger rates, eccentricities, and spins for field binary black holes driven to
merger by a third companion through the Lidov-Kozai mechanism. Using a population synthesis
approach, we model the creation of stellar-mass black hole triples across a range of different initial
conditions and stellar metallicities. We find that the production of triple-mediated mergers is enhanced
at low metallicities by a factor of ~ 100 due to the lower black hole natal kicks and reduced stellar mass
loss. These triples naturally yield heavy binary black holes with near-zero effective spins, consistent
with most of the mergers observed to date. This process produces a merger rate of between 2 and 25
Gpc-3yr-1 in the local universe, suggesting that the Lidov-Kozai mechanism can potentially explain
all of the low-spin, heavy black hole mergers observed by Advanced LIGO/Virgo. Finally, we show
that triples admit a unique eccentricity and spin distribution that will allow this model to be tested
in the near future.
The binary black hole mergers detected by Advanced LIGO/Virgo have shown no
evidence of large black hole spins. However, because LIGO/Virgo best measures the
effective combination of the two spins along the orbital angular momentum (Xeff), it is
difficult to distinguish between binaries with slowly-spinning black holes and binaries
with spins lying in the orbital plane. Here, we study the spin dynamics for binaries with
a distant black hole companion. For spins initially aligned with the orbital angular
momentum of the binary, we find that Xeff \freezes" near zero as the orbit decays
through the emission of gravitational waves. Through a population study, we show
that this process predominantly leads to merging black hole binaries with near-zero
Xeff. We conclude that if the detected black hole binaries were formed in triples, then
this would explain their low _eff without the need to invoke near-zero spins or initially
large spin-orbit angles.
Binaries within the sphere of influence of a massive black hole (MBH) in galactic nuclei are susceptible to
the Lidov-Kozai (LK) mechanism, which can drive orbits to high eccentricities and trigger strong interactions
within the binary such as the emission of gravitational waves (GWs), and mergers of compact objects. These
events are potential sources for GW detectors such as Advanced LIGO and VIRGO. The LK mechanism is
only effective if the binary is highly inclined with respect to its orbit around the MBH (within a few degrees of
90æ), implying low rates. However, close to an MBH, torques from the stellar cluster give rise to the process of
vector resonant relaxation (VRR). VRR can bring a low-inclination binary into an ?active? LK regime in which
high eccentricities and strong interactions are triggered in the binary. Here, we study the coupled LK-VRR
dynamics, with implications for LIGO and VIRGO GW sources. We carry out Monte Carlo simulations and
find that the merger fraction enhancement due to LK-VRR dynamics is up to a factor of < 10 for the lower
end of assumed MBH masses (M" = 104M), and decreases sharply with increasing M". We find that, even
in our most optimistic scenario, the baseline BH-BH merger rate is small, and the enhancement by LK-VRR
coupling is not large enough to increase the rate to well above the LIGO/VIRGO lower limit, 12Gpc?3 yr?1.
For the Galactic Center, the LK-VRR-enhanced rate is < 100 times lower than the LIGO/VIRGO limit, and for
M" = 104M, the rate barely reaches 12Gpc?3 yr?1.
Using proper motion measurements from Gaia DR2, we probe the origin of 26 previously known hypervelocity stars (HVSs) around the Milky Way. We find that a significant fraction of these stars have a high probability of originating close to the Milky Way centre, but there is one obvious outlier. HVS3 is highly likely to be coming almost from the centre of the Large Magellanic Cloud (LMC). During its closest approach, 21.1 +6.1 ?4.6 Myr ago, it had a relative velocity of 870 +69 ?66 kms ?1 with respect to the LMC. This large kick velocity is only consistent with the Hills mechanism, requiring a massive black hole at the centre of the LMC. This provides strong direct evidence that the LMC itself harbours a massive black hole of at least 4×10 3 ?10 4 M .
The Fermi Gamma-Ray Space Telescope has provided evidence for diffuse gamma-ray emission in the central parts of the Milky Way and the Andromeda galaxy. This excess has been interpreted either as dark-matter annihilation emission or as emission from thousands of millisecond pulsars (MSPs). We have recently shown that old massive globular clusters (GCs) may move toward the center of the Galaxy by dynamical friction and carry within them enough MSPs to account for the observed gamma-ray excess. In this Letter we revisit the MSP scenario for the Andromeda galaxy by modeling the formation and disruption of its GC system. We find that our model predicts gamma-ray emission ~2?3 times larger than for the Milky Way, but still nearly an order of magnitude smaller than the observed Fermi excess in the Andromeda. Our MSP model can reproduce the observed excess only by assuming ~8 times a larger number of old clusters than inferred from galaxy scaling relations. To explain the observations we require either that Andromeda deviates significantly from the scaling relations, or that a large part of its high-energy emission comes from additional sources.
In a star cluster with a sufficiently large escape velocity, black holes (BHs) that are produced by BH mergers can be retained, dynamically form new BH binaries, and merge again. This process can repeat several times and lead to significant mass growth. In this paper, we calculate the mass of the largest BH that can form through repeated BH mergers and determine how its value depends on the physical properties of the host cluster. We adopt an analytical model in which the energy generated by the black hole binaries in the cluster core is assumed to be regulated by the process of two-body relaxation in the bulk of the system. This principle is used to compute the hardening rate of the binaries and to relate this to the time-dependent global properties of the parent cluster. We demonstrate that in clusters with initial escape velocity s300kms?1 in the core and density s105M`pc?3`, repeated mergers lead to the formation of BHs in the mass range 100?105M`, populating any upper mass gap created by pair-instability supernovae. This result is independent of cluster metallicity and the initial BH spin distribution. We show that about 10 per cent of the present-day nuclear star clusters meet these extreme conditions, and estimate that BH binary mergers with total mass s100M` should be produced in these systems at a maximum rate H0.05Gpc?3yr?1`, corresponding to one detectable event every few years with Advanced LIGO/Virgo at design sensitivity.
Galactic nuclei are often found to contain young stellar populations and, in most cases, a central supermassive black hole (SMBH). Most known massive stars are found in binaries or higher-multiplicity systems, and in a galactic nucleus the gravitational interaction with the SMBH can affect their long-term evolution. In this paper, we study the orbital evolution of stellar binaries near SMBHs using high precision N-body simulations, and including tidal forces and Post-Newtonian corrections to the motion. We focus on the Lidov-Kozai (LK) effect induced by the SMBH on massive star binaries. We investigate how the properties of the merging binaries change with varying the SMBH mass, the slope of the initial mass function, the distributions of the binary orbital parameters and the efficiency in energy dissipation in dissipative tides. We find that the fraction of merging massive binary stars is in the range <4%?15% regardless of the details of the initial distributions of masses and orbital elements. For a Milky Way-like nucleus, we find a typical rate of binary mergers H1.4×10?7yr?1`. The merger products of massive binaries can be rejuvenated blue-straggler stars, more massive than each of their original progenitors, and G2-like objects. Binary systems that survive the LK cycles can be source of X-rays and gravitational waves, observable with present and upcoming instruments.
The coalescence of massive black hole binaries (BHBs) in galactic mergers is the primary
source of gravitational waves (GWs) at low frequencies. Current estimates of GW detection
rates for the Laser Interferometer Space Antenna and the Pulsar Timing Array vary by three
orders of magnitude. To understand this variation, we simulate the merger of equal-mass,
eccentric, galaxy pairs with central massive black holes and shallow inner density cusps. We
model the formation and hardening of a central BHB using the Fast Multiple Method as a
force solver, which features a O¹Nº scaling with the number N of particles and obtains results
equivalent to direct-summation simulations. At N ý 5ý105, typical for contemporary studies,
the eccentricity of the BHBs can vary significantly for different random realisations of the
same initial condition, resulting in a substantial variation of the merger timescale. This scatter
owes to the stochasticity of stellar encounters with the BHB and decreases with increasing N.
We estimate that N ý 107 within the stellar half-light radius suffices to reduce the scatter in
the merger timescale to ý 10%. Our results suggest that at least some of the uncertainty in
low-frequency GW rates owes to insufficient numerical resolution.