arXiv subjects

Roberto Capuzzo-Dolcetta

Publications and source records attributed to Roberto Capuzzo-Dolcetta.

At least 55 records · Page 3Linked to original sources

arXiv2016

Scaling Correlations Among Central Massive Objects and Their Host Galaxies

The central regions of galaxies show the presence of super massive black holes and/or very dense stellar clusters. Both such objects seem to follow similar host-galaxy correlations, suggesting that they are members of the same family of Compact Massive Objects. Here we investigate a huge data collection of Compact Massive Objects properties to correlate them with absolute magnitude, velocity dispersion and mass of their host galaxies. We draw also some preliminary astrophysical conclusions.

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arXiv2016

Will a nuclear stellar disk form in the galaxy Henize 2-10?

We present results of a set of $N$-body simulations to model the future evolution of the 11 young massive clusters hosted in the central region of the dwarf starburst galaxy Henize 2-10, which contains at its center a massive black hole with a mass $M_{\rm BH} \simeq 2\times 10^6$ M$_\odot$. Nuclear star clusters are present in a great quantity of galaxies of mass similar to Henize 2-10. Our results \citep{ASCD15} show that the orbital decay and merging of the Henize 2-10 clusters will likely lead to the formation of a nuclear star cluster with mass $M_{\rm NSC} \simeq 4-6 \times 10^6$ M$_\odot$ and effective radius $r_{\rm NSC} \simeq 4.1$ pc. Additionally, we found that this mechanism can lead to the formation of disky structures with global properties similar to those of nuclear stellar disks, which reside in many "middle-weight" galaxies. This work confirms and enlarge recent results \citep{ASCDS15} that indicate how nuclear star clusters and super massive black holes are only partially correlated, since the formation process of nuclear star clusters is poorly affected by a black hole of the size of that in Henize 2-10. A new result is that nuclear star clusters and nuclear stellar disks may share the same formation path.

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arXiv2016

The dearth of nuclear star clusters in bright galaxies

We investigate the interaction of a massive globular cluster (GC) with a super massive black hole (SMBH), located at the centre of its host galaxy, by means of direct N-body simulations. The results show that tidal distortions induced by the stellar background and the SMBH act on a time shorter than that of dynamical friction decay for a $10^6$ M$_\odot$ GC whenever the SMBH mass exceeds $\sim 10^8$ M$_\odot$ . This implies an almost complete dissolution of the infalling GC before it reaches the inner region ($\lesssim 5$ pc) of the parent galaxy. The generalization of this result to a larger sample of infalling GCs shows that such destructive process may prevent the formation and growth of a bright galactic nucleus. Another interesting, serendipitous, result we obtained is that the close interaction between the SMBH and the GC produces a "wave" of stars that escape from the cluster and, in a fraction, even from the whole galaxy.

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arXiv2015

High velocity stars from the interaction of a globular cluster and a massive black hole binary

High velocity stars are stars moving at velocities so high to require an acceleration mechanism involving binary systems or the presence of a massive central black hole. In the frame of a galaxy hosting a supermassive black hole binary (of total mass $10^8$ M$_\odot$), we investigated a mechanism for the production of high velocity stars due to the close interaction between a massive and orbitally decayed globular cluster and the super massive black hole binary. Some stars of the cluster acquire high velocities by conversion of gravitational energy into kinetic energy deriving from their interaction with the black hole binary. After the interaction, few stars reach a velocity sufficient to overcome the galactic gravitational well, while some of them are just stripped from the globular cluster and start orbiting around the galactic centre.

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arXiv2015

Super Massive Black Holes and the Origin of High-Velocity Stars

The origin of high velocity stars observed in the halo of our Galaxy is still unclear. In this work we test the hypothesis, raised by results of recent high precision $N$-body simulations, of strong acceleration of stars belonging to a massive globular cluster orbitally decayed in the central region of the host galaxy where it suffers of a close interaction with a super massive black hole, which, for these test cases, we assumed $10^8$ M$_\odot$ in mass.

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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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arXiv2015

The interaction between supermassive black holes and globular clusters

Almost all galaxies along the Hubble sequence host a compact massive object (CMO) in their center. The CMO can be either a supermassive black hole (SMBH) or a very dense stellar cluster, also known as nuclear star cluster (NSC). Generally, heavier galaxies (mass $\gtrsim 10^{11}\mathrm{M}_{\odot}$ ) host a central SMBH while lighter show a central NSC. Intermediate mass hosts, instead, contain both a NSC and a SMBH. One possible formation mechanisms of a NSC relies on the dry-merger (migratory) scenario, in which globular clusters (GCs) decay toward the center of the host galaxy and merge. In this framework, the absence of NSCs in high-mass galaxies can be imputed to destruction of the infalling GCs by the intense tidal field of the central SMBH. In this work, we report preliminary results of N-body simulations performed using our high-resolution, direct, code HiGPUs, to investigate the effects of a central SMBH on a single GC orbiting around it. By varying either the mass of the SMBH and the mass of the host galaxy, we derived an upper limit to the mass of the central SMBH, and thus to the mass of the host, above which the formation of a NSC is suppressed.

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arXiv2014

The Globular Cluster Migratory Origin of Nuclear Star Clusters

Nuclear Star Clusters (NSCs) are often present in spiral galaxies as well as resolved Stellar Nuclei (SNi) in elliptical galaxies centres. Ever growing observational data indicate the existence of correlations between the properties of these very dense central star aggregates and those of host galaxies, which constitute a significant constraint for the validity of theoretical models of their origin and formation. In the framework of the well known 'migratory and merger' model for NSC and SN formation, in this paper we obtain, first, by a simple argument the expected scaling of the NSC/SN mass with both time and parent galaxy velocity dispersion in the case of dynamical friction as dominant effect on the globular cluster system evolution. This generalizes previous results by \cite{TrOsSp} and is in good agreement with available observational data showing a shallow correlation between NSC/SN mass and galactic bulge velocity dispersion. Moreover, we give statistical relevance to predictions of this formation model, obtaining a set of parameters to correlate with the galactic host parameters. We find that the correlations between the masses of NSCs in the migratory model and the global properties of the hosts reproduce quite well the observed correlations, supporting the validity of the migratory-merger model. In particular, one important result is the flattening or even decrease of the value of the NSC/SN mass obtained by the merger model as function of the galaxy mass for high values of the galactic mass, i.e. $\gtrsim 3\times 10^{11}$M$_\odot$, in agreement with some growing observational evidence.

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arXiv2014

Dynamical friction in cuspy galaxies

In this paper we treat the problem of the dynamical friction decay of a massive object moving in an elliptical galaxy with a cuspidal inner distribution of the mass density. We present results obtained by both self-consistent, direct summation, N-body simulations, as well as by a new semi-analytical treatment of dynamical friction valid in such cuspy central regions of galaxies. A comparison of these results indicates that the proposed semi-analytical approximation is the only reliable in cuspy galactic central regions, where the standard Chandrasekhar's local approximation fails, and, also, gives estimates of decay times that are correct at 1% respect to those given by N-body simulations. The efficiency of dynamical friction in cuspy galaxies is found definitively higher than in core galaxies, especially on more radially elongated satellite orbits. As another relevant result, we find a proportionality of the dynamical friction decay time to the -0.67 power of the satellite mass, M, shallower than the standardly adopted 1/M dependence.

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arXiv2013

A Performance Comparison of Different Graphics Processing Units Running Direct N-Body Simulations

Hybrid computational architectures based on the joint power of Central Processing Units and Graphic Processing Units (GPUs) are becoming popular and powerful hardware tools for a wide range of simulations in biology, chemistry, engineering, physics, etc.. In this paper we present a comparison of performance of various GPUs available on market when applied to the numerical integration of the classic, gravitational, N-body problem. To do this, we developed an OpenCL version of the parallel code (HiGPUs) to use for these tests, because this version is the only apt to work on GPUs of different makes. The main general result is that we confirm the reliability, speed and cheapness of GPUs when applied to the examined kind of problems (i.e. when the forces to evaluate are dependent on the mutual distances, as it happens in gravitational physics and molecular dynamics). More specifically, we find that also the cheap GPUs built to be employed just for gaming applications are very performant in terms of computing speed also in scientific applications and, although with some limitations in central memory and in bandwidth, can be a good choice to implement a machine for scientific use at a very good performance to cost ratio.

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arXiv2013

Galactic Nuclear Cluster Formation Via Globular Cluster Mergers

We apply the idea that dense stellar systems in the central region of galaxies are formed via globular cluster mergers to the formation of the nuclear star cluster of the Milky Way, where a massive black hole is present. Our high precision N-body simulations show a good fit to the observational characteristics of the Milky Way nuclear cluster, giving further reliability to the so called migratory model for the formation of compact systems in the inner galaxy regions keywords galaxies: nuclei - galaxies: black holes - galaxies: globular clusters - galaxies: Milky Way - N-body: simulations

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arXiv2012

A study of low-energy transfer orbits to the Moon: towards an operational optimization technique

In the Earth-Moon system, low-energy orbits are transfer trajectories from the earth to a circumlunar orbit that require less propellant consumption when compared to the traditional methods. In this work we use a Monte Carlo approach to study a great number of such transfer orbits over a wide range of initial conditions. We make statistical and operational considerations on the resulting data, leading to the description of a reliable way of finding "optimal" mission orbits with the tools of multi-objective optimization.

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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

The Formation of the Milky Way Nuclear Cluster

Nuclear Star Clusters are observed at the center of many galaxies. In particular in the center of the Milky Way the Nuclear Star Cluster coexists with a cen- tral supermassive black hole. The origin of these clusters is still unknown; a possible formation mechanism is the decay of massive globular clusters driven inward to the galactic center by dynamical friction and their subsequent merging. By investigating this scenario by means of sophisticated N-body simulations we found that this process could lead to a final product which actually shows many of the observed features of the Milky Way Nuclear Star Cluster.

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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.

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arXiv2009

Globular Cluster System erosion in elliptical galaxies

In this paper we analyze data of 8 elliptical galaxies in order to study the difference between their globular cluster systems (GCSs) radial distributions and those of the galactic stellar component. In all the galaxies studied here the globular cluster system density profile is significantly flatter toward the galactic centre than that of stars. If this difference is interpreted as a depauperation of the initial GC population, the estimated number of missing globular clusters is significant, ranging from 21% to 71% of their initial population abundance in the eight galaxies examined. The corresponding mass lost to the central galactic region is 7x10^7-1.85x10^9 solar masses. All this mass carried toward central galactic regions have likely had an important feedback on the innermost galactic region, including its violent transient activity (AGN) and local massive black hole formation and growth.

astro-ph.CO↗

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.

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arXiv2009

High performance computing for classic gravitational N-body systems

The role of gravity is crucial in astrophysics. It determines the evolution of any system, over an enormous range of time and space scales. Astronomical stellar systems as composed by N interacting bodies represent examples of self-gravitating systems, usually treatable with the aid of newtonian gravity but for particular cases. In this note I will briefly discuss some of the open problems in the dynamical study of classic self-gravitating N-body systems, over the astronomical range of N. I will also point out how modern research in this field compulsorily requires a heavy use of large scale computations, due to the contemporary requirement of high precision and high computational speed.

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