Fabio is an STFC E. Rutherford fellow in the astrophysics group. Previously, he was a postdoctoral fellow at the Canadian Institute for Theoretical Astrophysics (CITA), and a CIERA postdoctoral fellow at Northwestern University. Prior to that, he received a PhD in Astrophysical Sciences and Technologies at the Rochester Institute of Technology (with David Merritt), and a Master degree in astrophysics from the university of Rome La Sapienza. His main research interests include stellar dynamics near supermassive black holes and computational astrophysics, formation of gravitational wave sources, and dynamics of exoplanet systems.
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.
with near-zero effective spin, Monthly Notices of the Royal Astronomical Society: Letters 480 (1) pp. L58-L62 Oxford University Press
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.
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 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 M" = 104M, the rate barely reaches 12Gpc?3 yr?1.
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.