arXiv subjects

Fabio Antonini

Publications and source records attributed to Fabio Antonini.

72 records · Page 4Linked to original sources

arXiv2015

The final-parsec problem in the collisionless limit

A binary supermassive black hole loses energy via ejection of stars in a galactic nucleus, until emission of gravitational waves becomes strong enough to induce rapid coalescence. Evolution via the gravitational slingshot requires that stars be continuously supplied to the binary, and it is known that in spherical galaxies the reservoir of such stars is quickly depleted, leading to stalling of the binary at parsec-scale separations. Recent N-body simulations of galaxy mergers and isolated nonspherical galaxies suggest that this stalling may not occur in less idealized systems. However, it remains unclear to what degree these conclusions are affected by collisional relaxation, which is much stronger in the numerical simulations than in real galaxies. In this study, we present a novel Monte Carlo method that can efficiently deal with both collisional and collisionless dynamics, and with galaxy models having arbitrary shapes. We show that without relaxation, the final-parsec problem may be overcome only in triaxial galaxies. Axisymmetry is not enough, but even a moderate departure from axisymmetry is sufficient to keep the binary shrinking. We find that the binary hardening rate is always substantially lower than the maximum possible, "full-loss-cone" rate, and that it decreases with time, but that stellar-dynamical interactions are nevertheless able to drive the binary to coalescence on a timescale <=1 Gyr in any triaxial galaxy.

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arXiv2015

The imprint of massive black-hole mergers on the correlation between nuclear clusters and their host galaxies

A literature compilation of nuclear star cluster (NSC) masses is used to study the correlation between global and nuclear properties. A comparison of observational data to the predictions of semi-analytical galaxy formation models places constraints on the co-evolution of NSCs, massive black holes (MBHs) and host galaxies. Both data and theoretical predictions show an increased scatter in the NSC scaling correlations at high galaxy masses, and we show that this is due to the progressively more efficient ejection of stars from NSCs caused by MBH binaries in more massive stellar spheroids. Our results provide a natural explanation of why in nucleated galaxies hosting a MBH, the ratio (M_NSC+M_MBH)/M_bulge (with M_bulge the host spheroid's mass) shows significantly less scatter than M_NSC/M_bulge, and suggest that the formation of MBHs and NSCs are not mutually exclusive, as also supported by observations of co-existing systems. Both MBHs and NSCs represent generic products of galaxy formation, with NSCs being destroyed or modified by the merger evolution of their companion MBHs.

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arXiv2015

Henize 2-10: the ongoing formation of a nuclear star cluster around a massive black hole

The central region of the galaxy Henize 2-10 has a central black hole (BH) with a mass of about $2\times 10^6$ M$_\odot$. While this black hole does not appear to coincide with any central stellar over density, it is surrounded by 11 young massive clusters with masses above $10^5$ M$_\odot$. The availability of high quality data on the structure of the galaxy and the age and mass of the clusters provides excellent initial conditions for studying the dynamical evolution of Henize 2-10's nucleus. Here we present a set of $N$-body simulations of the central clusters and black hole to understand whether and how they will merge to form a nuclear star cluster. Nuclear star clusters (NSCs) are present in a majority of galaxies with stellar mass similar to Henize 2-10. Despite the results depend on the choice of initial conditions, we find that a NSC with mass $M_{NSC}\simeq 4-6\times 10^6$ M$_\odot$ and effective radius $r_{NSC}\simeq 2.6-4.1$ pc will form within $0.2$ Gyr. This work is the first showing, in a realistic realization of the host galaxy and its star cluster system, that the formation of a bright nucleus is a process that can happen after the formation of a central massive BH leading to a composite NSC+BH central system. The merging process of the clusters does not affect significantly the kinematics of the BH, whose motion, after the globular cluster merger, is limited to a $\sim 1$ pc oscillation at less than $2$ kms$^{-1}$ speed.

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arXiv2014

Secular Evolution Of Binaries Near Massive Black Holes: Formation of compact binaries, merger/collision products and G2-like objects

Here we discuss the evolution of binaries around MBH in nuclear stellar clusters. We focus on their secular evolution due to the perturbation by the MBH, while simplistically accounting for their collisional evolution. Binaries with highly inclined orbits in respect to their orbit around the MBH are strongly affected by secular processes, which periodically change their eccentricities and inclinations (e.g. Kozai--Lidov cycles). During periapsis approach, dissipative processes such as tidal friction may become highly efficient, and may lead to shrinkage of a binary orbit and even to its merger. Binaries in this environment can therefore significantly change their orbital evolution due to the MBH third-body perturbative effects. Such orbital evolution may impinge on their later stellar evolution. Here we follow the secular dynamics of such binaries and its coupling to tidal evolution, as well as the stellar evolution of such binaries on longer time-scales. We find that stellar binaries in the central parts of NSCs are highly likely to evolve into eccentric and/or short period binaries, and become strongly interacting binaries either on the main sequence (at which point they may even merge), or through their later binary stellar evolution. The central parts of NSCs therefore catalyze the formation and evolution of strongly interacting binaries, and lead to the enhanced formation of blue stragglers, X-ray binaries, gravitational wave sources and possible supernova progenitors. Induced mergers/collisions may also lead to the formation of G2-like cloud-like objects such as the one recently observed in the Galactic center.

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arXiv2014

On the distribution of stellar remnants around massive black holes: slow mass segregation, star cluster inspirals and correlated orbits

We study the long term dynamical evolution of stellar mass black holes (BHs) at the Galactic center (GC) and put constraints on their number and central mass distribution. Models of the GC are considered that have not yet achieved a steady state under the influence of random gravitational encounters. Contrary to some recent claims that mass-segregation can rapidly rebuild a density cusp in the stars, we find that time scales associated with cusp regrowth are longer than the Hubble time. These results cast doubts on standard models that postulate high densities of BHs near the GC and motivate studies that start from initial conditions which correspond to well-defined physical models. For the first time, we consider the distribution of BHs in a dissipationless formation model for the Milky Way nuclear cluster (NC), in which massive stellar clusters merge in the GC to form a nucleus. We simulate the successive inspiral of massive clusters containing an inner dense cluster of BHs. The pre-existing mass segregation is not completely erased as the clusters are disrupted by the massive black hole tidal field. As a result, after 12 inspiral events a NC forms in which the BHs have higher central densities than the stars. After evolving the model for 5-10 Gyr, the BHs do form a steep central cusp, while the stellar distribution maintains properties that resemble those of the Milky Way NC. Finally, we investigate the effect of BH perturbations on the motion of the GC S-stars, as a means of constraining the number of the perturbers. We find that reproducing the S-star orbital distribution requires >~1000 BHs within 0.1 pc of Sgr A*. A dissipationless formation scenario for the Milky Way NC is consistent with this lower limit and therefore could reconcile the need for high central densities of BHs (to explain the orbits of the S-stars), with the missing-cusp problem of the GC giant star population.

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arXiv2014

The final-parsec problem in non-spherical galaxies revisited

We consider the evolution of supermassive black hole binaries at the center of spherical, axisymmetric, and triaxial galaxies, using direct N-body integrations as well as analytic estimates. We find that the rates of binary hardening exhibit a significant N-dependence in all the models, at least for N in the investigated range of 10^5<=N<=10^6. Binary hardening rates are also substantially lower than would be expected if the binary loss cone remained full, as it would be if the orbits supplying stars to the binary were being efficiently replenished. The difference in binary hardening rates between the spherical and nonspherical models is less than a factor of two even in the simulations with the largest N. By studying the orbital populations of our models, we conclude that the rate of supply of stars to the binary via draining of centrophilic orbits is indeed expected to be much lower than the full-loss-cone rate, consistent with our simulations. We argue that the binary's evolution in the simulations is driven in roughly equal amounts by collisional and collisionless effects, even at the highest N-values currently accessible. While binary hardening rates would probably reach a limiting value for large N, our results suggest that we cannot approach that rate with currently available algorithms and computing hardware. The extrapolation of results from N-body simulations to real galaxies is therefore not straightforward, casting doubt on recent claims that triaxiality or axisymmetry alone are capable of solving the final-parsec problem in gas-free galaxies.

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arXiv2013

Black hole triple dynamics: breakdown of the orbit average approximation and implications for gravitational wave detections

Coalescing black hole (BH) binaries forming in the dense core of globular clusters (GCs) are expected to be one the brightest sources of gravitational wave (GW) radiation for the next generation of ground-based laser interferometers. Favorable conditions for merger are initiated by the Kozai resonance in which the gravitational interaction with a third distant object, typically another BH, induces quasi-periodic variations of the inner BH binary eccentricity. In this paper we perform high precision N-body simulations of the long term evolution of hierarchical BH triples and investigate the conditions that lead to the merging of the BH binary and the way it might become an observable source of GW radiation. We find that the secular orbit average treatment, adopted in previous works, does not reliably describe the dynamics of these systems if the binary is orbited by the outer BH on a highly inclined orbit at a moderate distance. During the high eccentricity phase of a Kozai cycle the torque due to the outer BH can drive the binary to extremely large eccentricities in a fraction of the binary's orbital period. This occurs before relativistic terms become important to the evolution and allows the binary GW signal to reach large GW frequencies (>~10 Hz) at high eccentricities. We show that ~50 % of coalescing BH binaries driven by the Kozai mechanism in GCs will have eccentricities larger than 0.1, with 10 % of them being extremely eccentric, (1-e)<~10^-5, when they first chirp in the frequency band of ground based laser interferometers. This implies that a large fraction of such GW sources could be missed if conventional quasi-circular templates are used for analysis of GW detectors data. The efficient detection of all coalescing BH binaries in GCs will therefore require template banks of eccentric inspiral waveforms for matched-filtering and dedicated search strategies.

astro-ph.HE↗

arXiv2013

Relativity and the evolution of the Galactic center S-star orbits

We consider the orbital evolution of the S-stars, the young main-sequence stars near the supermassive black hole (SBH) at the Galactic center (GC), and put constraints on competing models for their origin. Our analysis includes for the first time the joint effects of Newtonian and relativistic perturbations to the motion, including the dragging of inertial frames by a spinning SBH as well as torques due to finite-N asymmetries in the field-star distribution (resonant relaxation, RR). The evolution of the S-star orbits is strongly influenced by the Schwarzschild barrier (SB), the locus in the (E,L) plane where RR is ineffective at driving orbits to higher eccentricities. Formation models that invoke tidal disruption of binary stars by the SBH tend to place stars below (i.e., at higher eccentricities than) the SB; some stars remain below the barrier, but most stars are able to penetrate it, after which they are subject to RR and achieve a nearly thermal distribution of eccentricities. This process requires roughly 50 Myr in nuclear models with relaxed stellar cusps, or >~10 Myr, regardless of the initial distribution of eccentricities, in nuclear models that include a dense cluster of 10 M_Sun black holes. We find a probability of <~1% for any S-star to be tidally disrupted by the SBH over its lifetime.

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arXiv2012

Origin and growth of nuclear star clusters around massive black holes

The centers of stellar spheroids are often marked by the presence of nucleated central regions, called nuclear star clusters (NSCs). The origin of NSCs is still unclear. Here we investigate the possibility that NSCs originate from the migration and merger of stellar clusters at the center of galaxies where a massive black hole (MBH) may sit. We show that the observed relation between NSC masses and the velocity dispersion of their host spheroids cannot be reconciled with a purely in-situ formation scenario. On the other hand, the observed relation appears to be in agreement with the predictions of the cluster merger model which also reproduces the observed relation between the size of NSCs and their total luminosity. We evolve through dynamical friction a population of stellar clusters in a model of a galactic bulge taking into account dynamical dissolution, starting from a power-law cluster initial mass function and a total mass in stellar clusters consistent with the cluster formation efficiency of the Milky Way (MW). The most massive clusters reach the center of the galaxy and merge to form a compact nucleus; after 10 Gyr, the resulting NSC has properties that are consistent with the observed distribution of stars in the MW NSC. When a MBH is included at the center of a galaxy, globular clusters are tidally disrupted during inspiral, resulting in NSCs with lower densities than those of NSCs forming in galaxies with no MBHs. We suggest this as a possible explanation for the lack of NSCs in galaxies containing MBHs more massive than ~10^8M_Sun. Finally, we investigate the orbital evolution of globular clusters in giant elliptical galaxies which are believed to always host a MBH at their center rather than a NSC. In these systems an additional mechanism can prevent a NSC from forming: the time for globular clusters to reach the center of the galaxy is much longer than the Hubble time.

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arXiv2012

Secular evolution of compact binaries near massive black holes: gravitational wave sources and other exotica

The environment near super massive black holes (SMBHs) in galactic nuclei contain a large number of stars and compact objects. A fraction of these are likely to be members of binaries. Here we discuss the binary population of stellar black holes and neutron stars near SMBHs and focus on the secular evolution of such binaries, due to the perturbation by the SMBH. Binaries with highly inclined orbits in respect to their orbit around the SMBH are strongly affected by secular Kozai processes, which periodically change their eccentricities and inclinations (Kozai-cycles). During periapsis approach, at the highest eccentricities during the Kozai-cycles, gravitational wave emission becomes highly efficient. Some binaries in this environment can inspiral and coalesce at timescales much shorter than a Hubble time and much shorter than similar binaries which do not reside near a SMBH. The close environment of SMBHs could therefore serve as catalyst for the inspiral and coalescence of binaries, and strongly affect their orbital properties. Such compact binaries would be detectable as gravitational wave (GW) sources by the next generation of GW detectors (e.g. advanced- LIGO). About 0.5% of such nuclear merging binaries will enter the LIGO observational window while on orbit that are still very eccentric (e>~0.5). The efficient gravitational wave analysis for such systems would therefore require the use of eccentric templates. We also find that binaries very close to the MBH could evolve through a complex dynamical (non-secular) evolution leading to emission of several GW pulses during only a few yrs (though these are likely to be rare). Finally, we note that the formation of close stellar binaries, X-ray binaries and their merger products could be induced by similar secular processes, combined with tidal friction rather than GW emission as in the case of compact object binaries.

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arXiv2012

Dissipationless Formation and Evolution of the Milky Way Nuclear Star Cluster

Abridged: In one widely discussed model for the formation of nuclear star clusters (NSCs), massive globular clusters spiral into the center of a galaxy and merge to form the nucleus. It is now known that at least some NSCs coexist with supermassive black holes (SBHs); this is the case, for instance, in the Milky Way (MW). In this paper, we investigate how the presence of a SMBH at the center of the MW impacts the merger hypothesis for the formation of its NSC. Starting from a model consisting of a low-density nuclear stellar disk and the SMBH, we use N-body simulations to follow the successive inspiral and merger of globular clusters. The clusters are started on circular orbits of radius 20 pc, and their initial masses and radii are set up in such a way as to be consistent with the galactic tidal field at that radius. The total accumulated mass by ~10 clusters is about 1.5x10^7 Solar masses. Each cluster is disrupted by the SMBH at a distance of roughly one parsec. The density profile that results after the final inspiral event is characterized by a core of roughly this radius, and an envelope with density that falls off as 1/r^2. These properties are similar to those of the MW NSC, with the exception of the core size, which in the MW is a little smaller. But by continuing the evolution of the model after the final inspiral event, we find that the core shrinks substantially via gravitational encounters in a time (when scaled to the MW) of 10 Gyr as the stellar distribution evolves toward a Bahcall-Wolf cusp. We also show that the luminosity function of the MW NSC is consistent with the hypothesis that a large fraction of the mass comes from (~10Gyr) old stars, brought in by globular clusters. We conclude that a model in which a large fraction of the mass of the MW NSC arose from infalling globular clusters is consistent with existing observational constraints.

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arXiv2011

Dynamical Friction around Supermassive Black Holes

The density of stars in galactic bulges is often observed to be flat or slowly rising inside the influence radius of the supermassive black hole (SMBH). Attributing the dynamical friction force to stars moving more slowly than the test body, as is commonly done, is likely to be a poor approximation in such a core since there are no stars moving more slowly than the local circular velocity. We have tested this prediction using large-scale N-body experiments. The rate of orbital decay never drops precisely to zero, because stars moving faster than the test body also contribute to the frictional force. When the contribution from the fast-moving stars is included in the expression for the dynamical friction force, and the changes induced by the massive body on the stellar distribution are taken into account, Chandrasekhar's theory is found to reproduce the rate of orbital decay remarkably well. However, this rate is still substantially smaller than the rate predicted by Chandrasekhar's formula in its most widely-used forms, implying longer time scales for inspiral. Motivated by recent observations that suggest a parsec-scale core around the Galactic center SMBH, we investigate the evolution of a population of stellar-mass black holes (BHs) as they spiral in to the center of the Galaxy. After ~10Gyr, we find that the density of BHs can remain substantially less than the density in stars at all radii; we conclude that it would be unjustified to assume that the spatial distribution of BHs at the Galactic center is well described by steady-state models. One consequence is that rates of capture of BHs by the SMBH at the Galactic center (EMRIs) may be much lower than in standard models. We finally study the orbital decay of satellite galaxies into the central region of giant ellipticals and discuss the formation of multiple nuclei and multiplet of black holes in such systems.

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arXiv2011

The Milky Way Nuclear Star Cluster

In the center of the Milky Way, as well as in many other galaxies, a compact star cluster around a very massive black hole is observed. One of the possible explanations for the formation of such Nuclear Star Clusters is based on the 'merging' of globular clusters in the inner galactic potential well. By mean of sophisticated N-body simulations, we checked the validity of this hypothesis and found that it may actually has been the one leading to the formation of the Milky Way Nuclear Star Cluster.

astro-ph.GA↗

arXiv2011

Tidal breakup of binary stars at the Galactic Center. II. Hydrodynamic simulations

In Paper I, we followed the evolution of binary stars as they orbited near the supermassive black hole (SMBH) at the Galactic center, noting the cases in which the two stars would come close enough together to collide. In this paper we replace the point-mass stars by fluid realizations, and use a smoothed-particle hydrodynamics (SPH) code to follow the close interactions. We model the binary components as main-sequence stars with initial masses of 1, 3 and 6 Solar masses, and with chemical composition profiles taken from stellar evolution codes. Outcomes of the close interactions include mergers, collisions that leave both stars intact, and ejection of one star at high velocity accompanied by capture of the other star into a tight orbit around the SMBH. For the first time, we follow the evolution of the collision products for many ($\gtrsim 100$) orbits around the SMBH. Stars that are initially too small to be tidally disrupted by the SMBH can be puffed up by close encounters or collisions, with the result that tidal stripping occurs in subsequent periapse passages. In these cases, mass loss occurs episodically, sometimes for hundreds of orbits before the star is completely disrupted. Repeated tidal flares, of either increasing or decreasing intensity, are a predicted consequence. In collisions involving a low-mass and a high-mass star, the merger product acquires a high core hydrogen abundance from the smaller star, effectively resetting the nuclear evolution "clock" to a younger age. Elements like Li, Be and B that can exist only in the outermost envelope of a star are severely depleted due to envelope ejection during collisions and due to tidal forces from the SMBH. In the absence of collisions, tidal spin-up of stars is only important in a narrow range of periapse distances, $r_t/2\lesssim r_per \lesssim r_t$ with $r_t$ the tidal disruption radius.

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arXiv2010

XMM-Newton detection of a transient X-ray source in the vicinity of V838 Monocerotis

We report the XMM-Newton/EPIC detection in 2008 March of a luminous (L_X ~ 10^32-33 erg/s), variable X-ray source in the vicinity (within ~6") of the enigmatic star V838 Mon, which underwent a spectacular outburst in early 2002. Spectral modeling of the XMM-Newton X-ray source indicates the presence of two plasma components with characteristic temperatures of TX ~ 2x10^6K and ~1.5x10^7K, attenuated by an absorbing column (N_H ~ 4 x 10^21cm^-2) that is consistent with the visual extinction measured toward V838 Mon (A_V ~ 2). No such luminous source was detected in the immediate vicinity of V838 Mon in Chandra/ACIS-S observations obtained about one year after outburst or, most recently, in 2010 January. The two XMM source spectral components appear to be marginally spatially resolved, with the spatial centroid of the hard component lying closer to (within ~2" of) the position of V838 Mon than the broad-band source or the soft source component; however, if there are two sources at or near V838 Mon, the Chandra nondetections would imply that both of them are variable. An alternative is that there is a single variable source, and that the apparent spatial separation may actually be due to photon-counting statistics or is perhaps instrumental in origin. We consider constraints placed by the X-ray detection and nondetections on a stellar merger model for the 2002 V838 Mon outburst, in which the spun-up merger product drives a powerful magnetic dynamo. Alternatively, the transient behavior of the X-ray source could indicate that the X-rays arose as a consequence of an interaction between V838 Mon's ejecta and its early-type (B3 V) companion.

astro-ph.SR↗

arXiv2009

Tidal break-up of binary stars at the Galactic center and its consequences

The tidal breakup of binary star systems by the supermassive black hole (SMBH) in the center of the galaxy has been suggested as the source of both the observed sample of hypervelocity stars (HVSs) in the halo of the Galaxy and the S-stars that remain in tight orbits around Sgr A*. Here, we use a post-Newtonian N-body code to study the dynamics of main-sequence binaries on highly elliptical bound orbits whose periapses lie close to the SMBH, determining the properties of ejected and bound stars as well as collision products. Unlike previous studies, we follow binaries that remain bound for several revolutions around the SMBH, finding that in the case of relatively large periapses and highly inclined binaries the Kozai resonance can lead to large periodic oscillations in the internal binary eccentricity and inclination. Collisions and mergers of the binary elements are found to increase significantly for multiple orbits around the SMBH, while HVSs are primarily produced during a binary's first passage. This process can lead to stellar coalescence and eventually serve as an important source of young stars at the galactic center.

astro-ph.GA↗

arXiv2009

A Counterpart to the Radial Orbit Instability in Triaxial Stellar Systems

Self-consistent solutions for triaxial mass models are highly non-unique. In general, some of these solutions might be dynamically unstable, making them inappropriate as descriptions of steady-state galaxies. Here we demonstrate for the first time the existence in triaxial galaxy models of an instability similar to the radial-orbit instability of spherical models. The instability manifests itself when the number of box orbits, with predominantly radially motions, is sufficiently large. N-body simulations verify that the evolution is due neither to chaotic orbits nor to departures of the model from self-consistency, but rather to a collective mode. The instability transforms the triaxial model into a more prolate, but still triaxial, configuration. Stable triaxial models are obtained when the mass contribution of radial orbits is reduced. The implications of our results for the shapes of dark-matter halos are discussed.

astro-ph.CO↗

arXiv2008

An Instability in Triaxial Stellar Systems

The radial-orbit instability is a collective phenomenon that has heretofore only been observed in spherical systems. We find that this instability occurs also in triaxial systems, as we checked by performing extensive N-body simulations whose initial conditions were obtained by sampling a self-consistent triaxial model of a cuspy galaxy composed of luminous and dark matter. N-body simulations show a time evolution of the galaxy that is not due to the development of chaotic motions but, rather, to the collective instability induced by an excess of box-like orbits. The instability quickly transforms such models into a more prolate configuration, with 0.64<b/a<0.77 and 0.6<c/a<0.7 for the dark halo and 0.64<b/a<0.77 and 0.59<c/a<0.67 for the luminous matter. Stable triaxial, cuspy galaxies with dark matter halos are obtained when the contribution of radially-biased orbits to the solution is reduced. These results constitute the first evidence of the radial-orbit instability in triaxial galaxy models.

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