arXiv subjects

Fabio Antonini

Publications and source records attributed to Fabio Antonini.

At least 55 records · Page 3Linked to original sources

arXiv2019

Black hole growth through hierarchical black hole mergers in dense star clusters: implications for gravitational wave detections

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 be formed through repeated mergers of stellar seed BHs 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 $\gtrsim 300\rm km\ s^{-1}$ in the core and density $\gtrsim 10^5\ M_\odot\rm pc^{-3}$, repeated mergers lead to the formation of BHs in the mass range $100-10^5 \,M_\odot$, 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 $10\%$ of the present-day nuclear star clusters meet these extreme conditions, and estimate that BH binary mergers with total mass $\gtrsim 100\,M _\odot$ should be produced in these systems at a maximum rate $\approx 0.05 \,\rm Gpc^{-3} yr^{-1}$, corresponding to one detectable event every few years with Advanced LIGO/VIRGO at design sensitivity. The contribution of globular clusters is likely to be negligible instead because the first BH merger remnant escapes following the relativistic kick. A possible connection of our results to the formation of massive BH seeds in galaxy nuclei and globular clusters is discussed.

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arXiv2019

Millisecond pulsars and the gamma-ray excess in Andromeda

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 may move towards the center of the Galaxy by dynamical friction and carry within them enough MSPs to account for the observed gamma-ray excess. In this paper we revisit the MSP scenario for the Andromeda galaxy, by modeling the formation and disruption of its globular cluster system. We find that our model predicts gamma-ray emission $\sim 2-3$ 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 $\sim 8$ times 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.

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arXiv2018

The impact of vector resonant relaxation on the evolution of binaries near a massive black hole: implications for gravitational wave sources

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 90 deg), 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_MBH = 1e4 MSun), and decreases sharply with increasing M_MBH. 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, 12 Gpc^{-3} yr^{-1}. For the Galactic Center, the LK-VRR-enhanced rate is ~100 times lower than the LIGO/VIRGO limit, and for M_MBH = 1e4 MSun, the rate barely reaches 12 Gpc^{-3} yr^{-1}.

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arXiv2018

A Triple Origin for the Heavy and Low-Spin Binary Black Holes Detected by LIGO/Virgo

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^-3 yr^-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.

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arXiv2018

Precessional dynamics of black hole triples: binary mergers with near-zero effective spin

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 ($χ_{\rm eff}$), 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 $χ_{\rm eff}$ "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 $χ_{\rm eff}$. We conclude that if the detected black hole binaries were formed in triples, then this would explain their low $χ_{\rm eff}$ without the need to invoke near-zero spins or initially large spin-orbit angles.

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arXiv2018

A hypervelocity star with a Magellanic origin

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 $4\times 10^3 -10^4 M_\odot$.

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arXiv2018

Disrupted Globular Clusters and the Gamma-Ray Excess in the Galactic Centre

The Fermi Large Area Telescope has provided the most detailed view toward the Galactic Centre (GC) in high-energy gamma rays. Besides the interstellar emission and point-source contributions, the data suggest a residual diffuse gamma-ray excess. The similarity of its spatial distribution with the expected profile of dark matter has led to claims that this may be evidence for dark matter particle annihilation. Here, we investigate an alternative explanation that the signal originates from millisecond pulsars (MSPs) formed in dense globular clusters and deposited at the GC as a consequence of cluster inspiral and tidal disruption. We use a semi-analytical model to calculate the formation, migration, and disruption of globular clusters in the Galaxy. Our model reproduces the mass of the nuclear star cluster and the present-day radial and mass distribution of globular clusters. For the first time, we calculate the evolution of MSPs from disrupted globular clusters throughout the age of the Galaxy and consistently include the effect of the MSP spin-down due to magnetic-dipole breaking. The final gamma-ray amplitude and spatial distribution are in good agreement with the Fermi observations and provide a natural astrophysical explanation for the GC excess.

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arXiv2017

Greatly enhanced merger rates of compact-object binaries in non-spherical nuclear star clusters

The Milky Way and a significant fraction of galaxies are observed to host a central Massive Black Hole (MBH) embedded in a non-spherical nuclear star cluster. We study the secular orbital evolution of compact-object binaries in these environments and characterize the excitation of extremely large eccentricities that can lead to mergers by gravitational radiation. We find that the eccentricity excitation occurs most efficiently when the nodal precession timescale of the binary's orbit around the MBH due to the non-spherical cluster becomes comparable (within a factor of ~10) to the timescale on which the binary is torqued by the MBH due to the Lidov-Kozai (LK) mechanism. We show that in this regime the perturbations due to the cluster increase the fraction of systems that reach extreme eccentricities ($1-e\sim10^{-4}-10^{-6}$) by a factor of ~10-100 compared to the idealized case of a spherical cluster, increasing the merger rates of compact objects by a similar factor. We identify two main channels that lead to this extreme eccentricity excitation: (i) chaotic diffusion of the eccentricities due to resonance overlap; (ii) cluster-driven variations of the mutual inclinations between the binary orbit and its center-of-mass orbit around the MBH, which can intensify the LK oscillations. We estimate that our mechanism can produce black hole-black hole and black hole-neutron star binary merger rates of up to $\approx 15$ $\rm{Gpc}^{-3}yr^{-1}$ and $\approx 0.4$ $\rm{Gpc}^{-3}yr^{-1}$, respectively. Thus, we propose the cluster-enhanced Lidov-Kozai mechanism as a new channel for the merger of compact-object binaries, competing with scenarios that invoke isolated binary evolution or dynamical formation in globular clusters.

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arXiv2017

Binary black hole mergers from field triples: properties, rates and the impact of stellar evolution

We consider the formation of binary black hole mergers through the evolution of field massive triple stars. In this scenario, favorable conditions for the inspiral of a black hole binary are initiated by its gravitational interaction with a distant companion, rather than by a common-envelope phase invoked in standard binary evolution models. We use a code that follows self-consistently the evolution of massive triple stars, combining the secular triple dynamics (Lidov-Kozai cycles) with stellar evolution. After a black hole triple is formed, its dynamical evolution is computed using either the orbit-averaged equations of motion, or a high-precision direct integrator for triples with weaker hierarchies for which the secular perturbation theory breaks down. Most black hole mergers in our models are produced in the latter non-secular dynamical regime. We derive the properties of the merging binaries and compute a black hole merger rate in the range (0.3- 1.3) Gpc^{-3}yr^{-1}, or up to ~2.5Gpc^{-3}yr^{-1} if the black hole orbital planes have initially random orientation. Finally, we show that black hole mergers from the triple channel have significantly higher eccentricities than those formed through the evolution of massive binaries or in dense star clusters. Measured eccentricities could therefore be used to uniquely identify binary mergers formed through the evolution of triple stars. While our results suggest up to ~10 detections per year with Advanced-LIGO, the high eccentricities could render the merging binaries harder to detect with planned space based interferometers such as LISA.

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arXiv2017

Dynamical friction and the evolution of Supermassive Black hole Binaries: the final hundred-parsec problem

The supermassive black holes originally in the nuclei of two merging galaxies will form a binary in the remnant core. The early evolution of the massive binary is driven by dynamical friction before the binary becomes "hard" and eventually reaches coalescence through gravitational wave emission. { We consider the dynamical friction evolution of massive binaries consisting of a secondary hole orbiting inside a stellar cusp dominated by a more massive central black hole.} In our treatment we include the frictional force from stars moving faster than the inspiralling object which is neglected in the standard Chandrasekhar's treatment. We show that the binary eccentricity increases if the stellar cusp density profile rises less steeply than $ρ\propto r^{-2}$. In cusps shallower than $ρ\propto r^{-1}$ the frictional timescale can become very long due to the deficit of stars moving slower than the massive body. Although including the fast stars increases the decay rate, low mass-ratio binaries ($q\lesssim 10^{-3}$) in sufficiently massive galaxies have decay timescales longer than one Hubble time. During such minor mergers the secondary hole stalls on an eccentric orbit at a distance of order one tenth the influence radius of the primary hole (i.e., $\approx 10-100\rm pc$ for massive ellipticals). We calculate the expected number of stalled satellites as a function of the host galaxy mass, and show that the brightest cluster galaxies should have $\gtrsim 1$ of such satellites orbiting within their cores. Our results could provide an explanation to a number of observations, which include multiple nuclei in core ellipticals, off-center AGNs and eccentric nuclear disks.

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arXiv2016

Secular dynamics of multiplanet systems: implications for the formation of hot and warm Jupiters via high-eccentricity migration

Hot Jupiters (HJs) are Jupiter-like planets that reside very closely to their host star, within $\sim 0.1\,\mathrm{AU}$. Their formation is not well understood. It is generally believed that they cannot have formed in situ, implying that some form of migration must have occurred after their initial formation. We study the production of HJs through secular evolution in multiplanet systems with three to five planets. In this variant of high-$e$ migration, the eccentricity of the orbit of the innermost planet is excited on secular time-scales, triggering orbital migration due to tidal dissipation. We use a secular dynamics code and carry out a population synthesis study. We find that HJs are only produced if the viscous time-scale is short ($\approx 0.014$ yr). In contrast, in up to $\approx 0.3$ of systems, the innermost planet is tidally disrupted. The orbital period distribution is peaked around 5 d, consistent with observations. The median HJ mass is $1\,M_\mathrm{J}$ with a maximum of $\approx 2 \, M_\mathrm{J}$, similar to observed HJs. Approximately 0.1 of the HJs have retrograde orbits with respect to the stellar spin. We do not find any warm Jupiters in our simulations, i.e. planets with semimajor axes between 0.1 and 1 AU.

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arXiv2016

Merging black hole binaries in galactic nuclei: implications for advanced-LIGO detections

Motivated by the recent detection of gravitational waves from the black hole binary merger GW150914, we study the dynamical evolution of black holes in galactic nuclei where massive star clusters reside. With masses of ~10^7M_Sun and sizes of only a few parsecs, nuclear star clusters are the densest stellar systems observed in the local universe and represent a robust environment where (stellar mass) black hole binaries can dynamically form, harden and merge. We show that due to their large escape speeds, nuclear star clusters can keep a large fraction of their merger remnants while also evolving rapidly enough that the holes can sink back to the central regions where they can swap in new binaries that can subsequently harden and merge. This process can repeat several times and produce black hole mergers of several tens of solar masses similar to GW150914 and up to a few hundreds of solar masses, without the need of invoking extremely low metallicity environments or implausible initial conditions. We use a semi-analytical approach to describe the formation and dynamics of black holes in massive star clusters. We find a black hole binary merger rate per volume from nuclear star clusters of ~1.5 Gpc^-3 yr^-1, implying up to a few tens of possible detections per year with Advanced LIGO. Our models suggest a local merger rate of 0.3- 1 Gpc^-3 yr^-1 for high mass black hole binaries similar to GW150914 (total mass >~ 50 M_Sun, redshift z< 0.3); a merger rate comparable to that of high mass black hole binaries that are dynamically assembled in globular clusters. Finally, we show that if all black holes receive high natal kicks, >~50km s^-1, then nuclear star clusters could dominate the local merger rate of binary black holes compared to the merger rate of similar binaries produced in either globular clusters or through isolated binary evolution.

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arXiv2016

Dynamical constraints on the origin of hot and warm Jupiters with close friends

Gas giants orbiting their host star within the ice line are thought to have migrated to their current locations from farther out. Here we consider the origin and dynamical evolution of observed Jupiters, focusing on hot and warm Jupiters with outer friends. We show that the majority of the observed Jupiter pairs (twenty out of twenty-four) will be dynamically unstable if the inner planet was placed at >~1AU distance from the stellar host. This finding is at odds with formation theories that invoke the migration of such planets from semi-major axes >~1AU due to secular dynamical processes (e.g., secular chaos, Lidov-Kozai oscillations) coupled with tidal dissipation. In fact, the results of N-body integrations show that the evolution of dynamically unstable systems does not lead to tidal migration but rather to planet ejections and collisions with the host star. This and other arguments lead us to suggest that most of the observed planets with a companion could not have been transported from further out through secular migration processes. More generally, by using a combination of numerical and analytic techniques we show that the high-e Lidov-Kozai migration scenario can only account for less than 10% of all gas giants observed between 0.1-1 AU. Simulations of multi-planet systems support this result. Our study indicates that rather than starting on highly eccentric orbits with orbital periods above one year, these "warm" Jupiters are more likely to have reached the region where they are observed today without having experienced significant tidal dissipation.

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arXiv2016

On the origins of enigmatic stellar populations in Local Group galactic nuclei

We consider the origins of enigmatic stellar populations in four Local Group galactic nuclei, specifically the Milky Way, M31, M32 and M33. These are centrally concentrated blue stars, found in three out of the four nuclear star clusters (NSCs) considered here. Their origins are unknown, but could include blue straggler (BS) stars, extended horizontal branch stars and young recently formed stars. Here, we calculate order-of-magnitude estimates for various collision rates, as a function of the host NSC environment and distance from the cluster centre. These rates are sufficiently high that BSs, formed via collisions between main sequence (MS) stars, could contribute non-negligibly ($\sim$ 1-10% in mass) to every surface brightness profile, with the exception of the Milky Way. Stellar evolution models show that the envelopes of red giant branch (RGB) stars must be nearly completely stripped to significantly affect their photometric appearance, which requires multiple collisions. Hence, the collision rates for individual RGB stars are only sufficiently high in the inner $\lesssim$ 0.1 pc of M31 and M32 for RGB destruction to occur. Collisions between white dwarfs and MS stars, which should ablate the stars, could offer a steady and significant supply of gas in every NSC in our sample. The gas could either fragment to form new stars, or accrete onto old MS stars already present. Thus, collisional processes could contribute significantly to the observed blue excesses in M31 and M33; future studies should be aimed at better constraining theoretical predictions to compliment existing and future observational data.

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arXiv2016

$N-$body dynamics of Intermediate mass-ratio inspirals in globular clusters

The intermediate mass-ratio inspiral of a stellar compact remnant into an intermediate mass black hole (IMBH) can produce a gravitational wave (GW) signal that is potentially detectable by current ground-based GW detectors (e.g., Advanced LIGO) as well as by planned space-based interferometers (e.g., eLISA). Here, we present results from a direct integration of the post-Newtonian $N$-body equations of motion describing stellar clusters containing an IMBH and a population of stellar-mass black holes (BHs) and solar mass stars. We take particular care to simulate the dynamics closest to the IMBH, including post-Newtonian effects up to order $2.5$. Our simulations show that the IMBH readily forms a binary with a BH companion. This binary is gradually hardened by transient 3-body or 4-body encounters, leading to frequent substitutions of the BH companion, while the binary's eccentricity experiences large amplitude oscillations due to the Lidov-Kozai resonance. We also demonstrate suppression of these resonances by the relativistic precession of the binary orbit. We find an intermediate mass-ratio inspiral in one of the 12 cluster models we evolved for $\sim 100$ Myr. This cluster hosts a $100 M_\odot$ IMBH embedded in a population of 32 $10M_\odot$ BH and 32,000 $1M_\odot$ stars. At the end of the simulation, after $\sim 100$ Myr of evolution, the IMBH merges with a BH companion. The IMBH--BH binary inspiral starts in the eLISA frequency window ($\gtrsim 1\rm mHz$) when the binary reaches an eccentricity $1-e\simeq 10^{-3}$. After $\simeq 10^5$ years the binary moves into the LIGO frequency band with a negligible eccentricity. We comment on the implications for GW searches, with a possible detection within the next decade.

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arXiv2016

Secular dynamics of hierarchical quadruple systems: the case of a triple system orbited by a fourth body

We study the secular gravitational dynamics of quadruple systems consisting of a hierarchical triple system orbited by a fourth body. These systems can be decomposed into three binary systems with increasing semimajor axes, binaries A, B and C. The Hamiltonian of the system is expanded in ratios of the three binary separations, and orbit-averaged. Subsequently, we numerically solve the equations of motion. We study highly hierarchical systems that are well described by the lowest-order terms in the Hamiltonian. We find that the qualitative behaviour is determined by the ratio $\mathcal{R}_0$ of the initial Kozai-Lidov (KL) time-scales of the binary pairs AB and BC. If $\mathcal{R}_0\ll 1$, binaries AB remain coplanar if this is initially the case, and KL eccentricity oscillations in binary B are efficiently quenched. If $\mathcal{R}_0\gg 1$, binaries AB become inclined, even if initially coplanar. However, there are no induced KL eccentricity oscillations in binary A. Lastly, if $\mathcal{R}_0\sim 1$, complex KL eccentricity oscillations can occur in binary A that are coupled with the KL eccentricity oscillations in B. Even if binaries A and B are initially coplanar, the induced inclination can result in very high eccentricity oscillations in binary A. These extreme eccentricities could have significant implications for strong interactions such as tidal interactions, gravitational wave dissipation, and collisions and mergers of stars and compact objects. As an example, we apply our results to a planet+moon system orbiting a central star, which in turn is orbited by a distant and inclined stellar companion or planet, and to observed stellar quadruples.

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arXiv2015

Black hole mergers and blue stragglers from hierarchical triples formed in globular clusters

Hierarchical triple-star systems are expected to form frequently via close binary-binary encounters in the dense cores of globular clusters. In a sufficiently inclined triple, gravitational interactions between the inner and outer binary can cause large-amplitude oscillations in the eccentricity of the inner orbit ("Lidov-Kozai cycles"), which can lead to a collision and merger of the two inner components. In this paper we use Monte Carlo models of dense star clusters to identify all triple systems formed dynamically and we compute their evolution using a highly accurate three-body integrator which incorporates relativistic and tidal effects. We find that a large fraction of these triples evolve through a non-secular dynamical phase which can drive the inner binary to higher eccentricities than predicted by the standard secular perturbation theory (even including octupole-order terms). We place constraints on the importance of Lidov-Kozai-induced mergers for producing: (i) gravitational wave sources detectable by Advanced LIGO (aLIGO), for triples with an inner pair of stellar black holes; and (ii) blue straggler stars, for triples with main-sequence-star components. We find a realistic aLIGO detection rate of black hole mergers due to the Lidov-Kozai mechanism of 1yr^-1, with about 20% of these having a finite eccentricity when they first chirp into the aLIGO frequency band. While rare, these events are likely to dominate among eccentric compact object inspirals that are potentially detectable by aLIGO. For blue stragglers, we find that the Lidov-Kozai mechanism can contribute only up to ~10% of their total numbers in globular clusters.

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arXiv2015

Co-evolution of nuclear star clusters, massive black holes and their host galaxies

Studying how nuclear star clusters (NSCs) form and how they are related to the growth of the central massive black holes (MBHs) and their host galaxies is fundamental for our understanding of the evolution of galaxies and the processes that have shaped their central structures. We present the results of a semi-analytical galaxy formation model that follows the evolution of dark matter halos along merger trees, as well as that of the baryonic components. This model allows us to study the evolution of NSCs in a cosmological context, by taking into account the growth of NSCs due to both dynamical friction-driven migration of stellar clusters and star formation triggered by infalling gas, while also accounting for dynamical heating from (binary) MBHs. We find that in-situ star formation contributes a significant fraction (up to ~80%) of the total mass of NSCs in our model. Both NSC growth through in-situ star formation and through star cluster migration are found to generate NSC -- host galaxy scaling correlations that are shallower than the same correlations for MBHs. We explore the role of galaxy mergers on the evolution of NSCs, and show that observational data on NSC -- host galaxy scaling relations provide evidence of partial erosion of NSCs by MBH binaries in luminous galaxies. We show that this observational feature is reproduced by our models, and we make predictions about the NSC and MBH occupation fraction in galaxies. We conclude by discussing several implications for theories of NSC formation.

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