Abstract: We present the results of detailed N-body simulations regarding the interaction of four massive globular clusters in the central region of a triaxial galaxy. The systems undergo a full merging event, producing a sort of `Super Star Cluster' (SSC) whose features are close to those of a superposition of the individual initial mergers. In contrast with other similar simulations, the resulting SSC structural parameters are located along the observed scaling relations of globular clusters. These findings seem to support the idea that a massive SSC may have formed in early phases of the mother galaxy evolution and contributed to the growth of a massive nucleus.

Abstract: Not Available

Abstract: We report the detection of a stellar density cusp and a velocity dispersion increase in the center of the globular cluster M54, located at the center of the Sagittarius dwarf galaxy (Sgr). The central line-of-sight velocity dispersion is 20.2 ￜ 0.7 km s<SUP>-1</SUP>, decreasing to 16.4 ￜ 0.4 km s<SUP>-1</SUP> at 2farcs5 (0.3 pc). Modeling the kinematics and surface density profiles as the sum of a King model and a point-mass yields a black hole mass of ~9400 M <SUB>sun</SUB>. However, the observations can alternatively be explained if the cusp stars possess moderate radial anisotropy. A Jeans analysis of the Sgr nucleus reveals a strong tangential anisotropy, probably a relic from the formation of the system.

Abstract: Preliminary results are presented about a fully self-consistent N-body simulation of a sample of four massive globular clusters in close interaction within the central region of a galaxy. The N-body representation (with N=1.5ï¿œ 10<SUP>6</SUP> particles in total) of both the clusters and the galaxy allows to include in a natural and self-consistent way dynamical friction and tidal interactions. The results confirm the decay and merging of globulars as a viable scenario for the formation/accretion of compact nuclear clusters. Specifically: i) the frictional orbital decay is ˜ 2 times faster than that predicted by the generalized Chandrasekhar formula; ii) the progenitor clusters merge in less than 20 galactic core-crossing time (t_b); iii) the NC configuration keeps a quasi-stable state at least within ˜ 70 t_b.

Abstract: Preliminary results are presented about a fully self-consistent N-body simulation of a sample of four massive globular clusters in close interaction within the central region of a galaxy. The N-body representation (with N=1.5ï¿œ 10<SUP>6</SUP> particles in total) of both the clusters and the galaxy includes in a natural and self-consistent way dynamical friction and tidal interactions. The results confirm the decay and merging of globulars as a viable scenario for the formation/accretion of compact nuclear clusters. Specifically: i) the frictional orbital decay is ˜ 2 times faster than that predicted by the generalized Chandrasekhar formula; ii) the progenitor clusters merge in less than 20 galactic core-crossing time (t_b); iii) the NC configuration keeps quasi-stable at least within ˜ 70 t_b.

Abstract: In this series of papers, we present the results of detailed N-body simulations of the interaction of a sample of four massive globular clusters in the inner region of a triaxial galaxy for two different sets of initial conditions that correspond to different initial density concentrations. A full merging of the clusters takes place, leading to a slowly evolving cluster that is quite similar to observed nuclear clusters. Actually, both the density and the velocity dispersion profiles match qualitatively, and also quantitatively after scaling to a larger number of merger globulars, with the observed features of many nucleated galaxies. In the case of dense initial clusters, the merger remnant shows a density profile more concentrated than that of the progenitors, with a central density higher than the sum of the progenitors' central densities and an effective radius compatible with observed nuclear values. These findings support the idea that a massive nuclear cluster may have formed in the early phases of the mother galaxy's evolution and led to the formation of a nucleus, which in many galaxies has a luminosity profile similar to that of an extended King model. A correlation with galactic nuclear activity is suggested.

Abstract: We present results of fully self-consistent N-body simulations of the motion of four globular clusters moving in the inner region of their parent galaxy. With regard to previous simplified simulations, we confirm merging and formation of an almost steady nuclear cluster, in a slightly shorter time. The projected surface density profile shows strong similarity to that of resolved galactic nuclei. This similarity reflects also in the velocity dispersion profile which exhibits a central colder component as observed in many nucleated galaxies.

Abstract: We have used a combination of HST high-resolution and ground-based wide-field observations of the Galactic globular cluster NGC 6388 to derive its center of gravity, projected density profile, and central surface brightness profile. While the overall projected profiles are well fit by a King model with an intermediate concentration (c=1.8) and a sizable core radius (r<SUB>c</SUB> = 7.2"), a significant power-law (with slope alpha=-0.2) deviation from flat-core behavior has been detected within the inner 1". These properties suggest the presence of a central intermediate-mass black hole. The observed profiles are well reproduced by a multimass isotropic, spherical model, including a black hole with a mass of ~5.7 ￜ 10<SUP>3</SUP> M<SUB>solar</SUB>. Based on observations with the NASA/ESA Hubble Space Telescope (HST), obtained at the Space Telescope Science Institute (STScI), which is operated by AURA, Inc., under NASA contract NAS5-26555. Also based on WFI observations collected at the European Southern Observatory (ESO), La Silla, Chile.

Abstract: We present the results of detailed N-body simulations of clusters moving in a realistic Milky Way (MW) potential. The strong interaction with the bulge and the disk of the Galaxy leads to the formation of tidal tails, emanating from opposite sides of the cluster. Some characteristic features in the morphology and orientation of these streams are recognized and interpreted. The tails have a complex morphology, particularly when the cluster approaches its apogalacticon, showing multiple ``arms'' in remarkable similarity to the structures observed around NGC 288 and Willman 1. Actually, the tails are generally good tracers of the cluster path quite far from the cluster center (>7-8 tidal radii), while on the smaller scale they are mainly pointing in the direction of the Galaxy center. In particular, the orientation of the inner part of the tails is highly correlated with the cluster orbital phase and the local orbital angular acceleration. This implies that, in general, the orbital path cannot be estimated directly from the orientation of the tails, unless a sufficient large field around the cluster is available.

Abstract: By means of a multimass isotropic and spherical model that includes the self-consistent treatment of a central intermediate-mass black hole (IMBH), the influence of this black hole on the morphological and physical properties of globular clusters is investigated. In agreement with recent numerical studies, it is found that a cluster (with mass M) hosting an IMBH (with mass M<SUB>•</SUB>) shows, outside the region of gravitational influence of the black hole, a core-like profile resembling a King profile with concentration c < 2, although with a slightly steeper behaviour in the core region. In particular, the core logarithmic slope is s <~ 0.25 for reasonably low IMBH masses (M<SUB>•</SUB> <~ 10<SUP>-2</SUP> M), and c decreases monotonically with M<SUB>•</SUB>. Completely power-law density profiles (similar to, for example, those of collapsed clusters) are admitted only in the presence of a black hole with an unrealistic M<SUB>•</SUB> > M. The mass range estimate 12s - 4.8 < log(M<SUB>•</SUB>/M) < -1.1c - 0.69, depending on the morphological parameters, is deduced by considering a wide range of models. Applying this estimate to a set of 39 globular clusters, it is found that NGC 2808, NGC 6388, M80, M13, M62, M54 and G1 (in M31) probably host an IMBH. For them, the scaling laws M<SUB>•</SUB> ~ 0.09(M/M<SUB>solar</SUB>)<SUP>0.7</SUP>M<SUB>solar</SUB> and M<SUB>•</SUB> ~ 50(sigma<SUB>obs</SUB>/km s<SUP>-1</SUP>)<SUP>1.2</SUP>M<SUB>solar</SUB> are identified from weighted least-squares fits. An important result of this `collective' study is that a strong correlation exists between the presence of an extreme blue horizontal branch and the presence of an IMBH. In particular, the presence of a central IMBH in M13 and NGC 6388 could explain why these clusters possess extreme horizontal-branch stars, in contrast to their `second parameter' counterparts M3 and 47 Tuc.

Abstract: In the construction of multimass King-Michie models of globular clusters, an approximated central energy equipartition between stars of different mass is usually imposed by scaling the velocity parameter of each mass class inversely with the stellar mass, as if the distribution function were isothermal. In this paper, this `isothermal approximation' has been checked and its consequences on the model parameters studied by a comparison with models including central energy equipartition correctly. It is found that, under the isothermal approximation, the `temperatures' of a pair of components can differ to a non-negligible amount for low concentration distributions. It is also found that, in general, this approximation leads to a significantly reduced mass segregation in comparison with that given under the exact energy equipartition at the centre. As a representative example, an isotropic three-component model fitting a given projected surface brightness and line-of-sight velocity dispersion profiles is discussed. In this example, the isothermal approximation gives a cluster envelope much more concentrated (central dimensionless potential W= 3.3) than under the true equipartition (W= 5.9 ￜ 10<SUP>-2</SUP>), as well as a higher mass function logarithmic slope. As a consequence, the inferred total mass (and then the global mass-to-light ratio) is a factor of 1.4 times lower than the correct value and the amount of mass in heavy dark remnants is 3.3 times smaller. Under energy equipartition, the fate of stars having a mass below a certain limit is to escape from the system. This limit is derived as a function of the mass and W of the component of giant and turn-off stars.

Abstract: The main topic of this paper is the investigation of the modes of interaction of globular clusters (GCs) moving in the inner part of a galaxy. This is tackled by means of high-resolution N-body simulations, whose first results are presented in this article. Our simulations dealt with primordial, very massive (of the order of 10<SUP>7</SUP> M<SUB>solar</SUB>) GCs that were able to decay, because of dynamical friction, into the inner regions of triaxial galaxies on a timescale much shorter than their internal relaxation time. To check the disruptive roles of both tidal forces and GC-GC collisions, their effects were maximized by considering clusters on quasi-radial orbits and choosing the initial conditions so as to give head-on collisions at each passage through the center. The available CPU resources allowed us to simulate clusters with different structural parameters and to follow them on quasi-radial orbits during eight passages across the center. The main findings are as follows: (1) clusters with an initial high-enough King concentration parameter (c>=1.2) preserve up to 50% of their initial mass, (2) the inner density distribution of the surviving clusters keeps a King model profile, (3) GC-GC collisions have a negligible effect compared with that caused by the passage through the galactic center, (4) the orbital energy dissipation due to the tidal interaction is of the same order as that caused by dynamical friction, (5) complex substructures like ``ripples'' and ``clumps'' are formed, as observed around real clusters. These findings support the validity of the hypothesis of merging of GCs in the galactic central region, with modes that deserve further careful investigation.

Abstract: Not Available

Abstract: In this paper, we present some preliminary results of numerical simulations of a globular cluster orbiting in the inner region of a triaxial galaxy. The formation and evolution of tidal tails around the cluster is discussed, together with their morphology, which shows clumpy substructures as observed, for example, in the galactic globular cluster Palomar 5.

Abstract: In order to investigate some relevant aspects of globular clusters formation we studied numerically the dynamics of a self-gravitating stellar system of the size of a typical globular cluster starting from non-equilibrium and 'cold' initial conditions. Such conditions are indeed those expected after the formation of the stellar system i.e. after the complete exhaustion of primordial gas through the gas-star phase transition which occurred on the star-formation time scale. Our (preliminary) results allow to draw some meaningful conclusions about the evolution of the distribution function toward a meta-stable state by mean of the quick 'violent relaxation' of the whole system (core formation mass segregation anisotropy...). To this virialized state follows the well known further evolution caused by star-star encounters. We underline that the initial 'violent' dynamics can be well investigated by mean of codes (like our own tree-code) which are efficient in the representation of the time evolution driven by the quick mean-field variations even if the individual 2-body close interactions (which are not really relevant in this phase) are not followed in detail.

Abstract: We present some results of numerical simulations of a globular cluster orbiting in the central region of a triaxial galaxy on a set of ``loop'' orbits. Tails start forming after about a quarter of the globular cluster orbital period and develop, in most cases, along the cluster orbit, showing clumpy substructures as observed, for example, in Palomar 5. If completely detectable, clumps can contain about 7000 M<SUB>solar</SUB> each, i.e., about 10% of the cluster mass at that epoch. The morphology of tails and clumps and the kinematical properties of stars in the tails are studied and compared with available observational data. Our finding is that the stellar velocity dispersion tends to level off at large radii, in agreement with that found for M15 and omega Cen.

Abstract: Astrocomp is a project developed by the INAF-Astrophysical Observatory of Catania, University of Roma La Sapienza and Enea in collaboration with Oneiros s.r.l. The project has the goal of building a web-based user-friendly interface which allows the international community to run some parallel codes on a set of high-performance computing (HPC) resources, with no need for specific knowledge about Unix and Operating Systems commands. Astrocomp provides CPU times, on parallel systems, available to the authorized user. The portal makes codes for astronomy available: FLY code, a cosmological code for studying three-dimensional collisionless self-gravitating systems with periodic boundary conditions [Becciani, Antonuccio, Comput. Phys. Comm. 136 (2001) 54]. ATD treecode, a parallel tree-code for the simulation of the dynamics of self-gravitating systems [Miocchi, Capuzzo Dolcetta, A&A 382 (2002) 758]. MARA a code for stellar light curves analysis [Rodonￜ et al., A&A 371 (2001) 174]. Other codes will be added to the portal in the future.

Abstract: Not Available

Abstract: Not Available

Abstract: In order to investigate some relevant aspects of globular clusters formation we studied numerically the dynamics of a self-gravitating stellar system of the size of a typical globular cluster starting from non-equilibrium and 'cold' initial conditions. Such conditions are indeed those expected after the formation of the stellar system i.e. after the complete exhaustion of primordial gas through the gas-star phase transition which occurred on the star-formation time scale. Our (preliminary) results allow to draw some meaningful conclusions about the evolution of the distribution function toward a meta-stable state by mean of the quick 'violent relaxation' of the whole system (core formation mass segregation anisotropy...). To this virialized state follows the well known further evolution caused by star-star encounters. We underline that the initial 'violent' dynamics can be well investigated by mean of codes (like our own tree-code) which are efficient in the representation of the time evolution driven by the quick mean-field variations even if the individual 2-body close interactions (which are not really relevant in this phase) are not followed in detail

Abstract: Astrocomp is a project developed by the Astrophysical Observatory of Catania, University of Roma La Sapienza and ENEA. The project goal is to build a web-based user-friendly interface which allows the international community to run some parellel codes on a set of high performance computing (HPC) resources. There' s no need for specific knowledge about Unix commands and Operating Systems. Astrocomp makes some CPU times, on large parallel platform, available to the referenced user .

Abstract: A practical utilization of the Astrocomp web portal is illustrated in the framework of the numerical simulations of Astrophysical systems. It is shown how to handle the various aspects related to the performing and managing of a typical N-body simulation. For this purpose, the features and usage of one of the parallel codes available in AstroComp, namely the ``treeATD'' code, are briefly described.

Abstract: The modern study of the dynamics of stellar systems requires the use of high-performance computers. Indeed, an accurate modelization of the structure and evolution of self-gravitating systems like planetary systems, open clusters, globular clusters and galaxies imply the evaluation of body-body interaction over the whole size of the structure, a task that is computationally very expensive, in particular when it is performed over long intervals of time. In this report we give a concise overview of the main problems of stellar systems simulations and present some exciting results we obtained about the interaction of globular clusters with the parent galaxy.

Abstract: We present the results of an extensive series of high-performance simulations of the evolution of self-gravitating systems with periodic boundary conditions. The main aim of the project is to investigate the role of gravitation and of initial conditions and boundary conditions into the following evolution toward a metastable equilibrium, in a way such to distinguish the role of the various ingredients in the overall dynamics. In particular, we compare the evolution of spatially infinite self-gravitating systems embedded in an expanding universe with that of systems in a static frame. We discuss the differences and the similarities in several statistical quantities, as the density profiles of clusters and the two point autocorrelation function.

Abstract: We describe a parallel version of our tree-code for the simulation of self-gravitating systems in Astrophysics. It is based on a dynamic and adaptive method for the domain decomposition, which exploits the hierarchical data arrangement used by the tree-code. It shows low computational costs for the parallelization overhead - less than 4% of the total CPU-time in the tests done - because the domain decomposition is performed ``on the fly'' during the tree-construction and the portion of the tree that is local to each processor ``enriches'' itself of remote data only when they are actually needed. The performance of an implementation of the parallel code on a Cray T3E is presented and discussed. They exhibit a very good behaviour of the speedup (=15 with 16 processors and 10<SUP>5</SUP> particles) and a rather low load unbalancing (<10% using up to 16 processors), achieving a high computation speed in the forces evaluation (>10<SUP>4</SUP> particles/sec with 8 processors). Supported by CINECA (http://www.cineca.it) and CNAA (http://cnaa.cineca.it) under Grant cnarm12a.

Abstract: We study gravitational clustering of mass points in three dimensions with random initial positions and periodic boundary conditions (no expansion) by numerical simulations. Correlation properties are well-defined in the system and a sort of thermodynamic limit can be defined for the transient regime of clustering. Structure formation proceeds along two paths: i) fluid-like evolution of density perturbations at large scales and ii) shift of the granular (non-fluid) properties from small to large scales. The latter mechanism finally dominates at all scales and it is responsible for the self-similar characteristics of the clustering.

Abstract: Self-gravitating systems have acquired growing interest in statistical mechanics, due to the peculiarities of the 1/r potential. Indeed, the usual approach of statistical mechanics cannot be applied to a system of many point particles interacting with the Newtonian potential, because of (i) the long-range nature of the 1/r potential and of (ii) the divergence at the origin. We study numerically the evolutionary behavior of self-gravitating systems with periodical boundary conditions, starting from simple initial conditions. We do not consider in the simulations additional effects as the (cosmological) metric expansion and/or sophisticated initial conditions, since we are interested whether and how gravity by itself can produce clustered structures. We are able to identify well-defined correlation properties during the evolution of the system, which seem to show a well-defined thermodynamic limit, as opposed to the properties of the ``equilibrium state''. Gravity-induced clustering also shows interesting self-similar characteristics.

Abstract: Some aspects of the dynamical evolution of a globular cluster as satellite of a galaxy still deserves a deep investigation. Here we study the effects produced by the tidal interaction of a globular cluster with the bulge as well as with a massive black hole sited in the central galactic region. Detailed studies of such dynamical mechanisms are rather hard to be performed by analytical means (unless heavy simplifying assumptions are made). For this reason we pesent here results obtained by our preliminary N-body simulations, which indicate that: 1) the time scale of the bulge tidal disruption is longer than 3ￜ10<SUP>8</SUP> yr for a globular cluster (mass 10<SUP>6</SUP>M<SUB>solar</SUB>, core radius 1 pc) moving in the inner kpc of a 10<SUP>9</SUP> M<SUB>solar</SUB> bulge (with core radius 200 pc); 2) the close tidal interaction between a massive black hole and a cluster moving along small quasi-circular orbits (with radius <20 pc) can give rise to a sort of friction which dissipates all the kinetic orbital energy, increasing the internal "temperature" of the cluster in less than 10<SUP>8</SUP> yr. Such simulations have been carried out with both a serial and a parallel "tree-code" (implemented on a CRAY T3E machine), using a leap-frog scheme with individual and variable time steps for the integration of particles' trajectories.

Abstract: The evolution of globular cluster systems in some galaxies can be cause of merging of globulars in the very central regions. This high stellar density favours the growth of a central nucleus via swallowing of surrounding stars. The infall of stars into a nuclear black hole is here shown to be, under certain conditions, not only source of electromagnetic radiation but also a significant source of gravitational waves.

Abstract: We report preliminary results on the parallelization of a tree-code for evaluating gravitational forces in N -body astrophysical systems. Our HPF/CRAFT implementation on a CRAY T3E machine attained an encouraging speed-up behavior, reaching a value of 75 with 128 processor elements (PEs). The speed-up tests regarded the evaluation of the forces among N =130,369 particles distributed scaling the actual distribution of a sample of galaxies.

Abstract: The authors present tests of comparison between their versions of the fast multipole algorithm (FMA) and the tree-code to evaluate gravitational forces in particle systems. They have optimized Greengard's original version of FMA allowing for a more efficient criterion of well-separation between boxes, to improve the adaptivity of the method (which is very important in highly inhomogeneous situations) and to permit the smoothing of gravitational interactions. The results of the tests indicate that the tree-code is 2 - 4 times faster than the FMA for clumped distributions and 3 - 9 times for homogeneous distributions, at least in the interval of N here investigated (N <= 2ï¿œ10<SUP>5</SUP>) and at the same level of accuracy (error ≡10<SUP>-3</SUP>). This order of accuracy is generally considered as the best compromise between CPU-time consumption and precision for astrophysical simulations. Moreover, the claimed linear dependence on N of the CPU-time of the FMA is not confirmed and the authors give a "theoretical" explanation for that.

Abstract: The decay of globular clusters to the centre of their mother galaxy corresponds to carrying a quantity of mass sufficient to sustain the gravitational activity of a small pre-existing nucleus and to accrete it in a significant way. This is due to both dynamical friction of field stars and tidal disruption by the compact nucleus. The results of the simplified model presented here show that the active galactic nuclei luminosity and lifetime depend on the characteristics of the globular cluster system and are quite insensitive to the nucleus' initial mass.

Abstract: The decay of globular clusters to the centre of their mother galaxy corresponds to carrying a quantity of mass sufficient to sustain the gravitational activity of a small pre-existing nucleus and to accrete it in a significant way. This is due to both dynamical friction of field stars and tidal disruption by the compact nucleus. The results of the simplified model presented here show that the active galactic nuclei luminosity and lifetime depend on the characteristics of the globular cluster system and are quite insensitive to the nucleus' initial mass.

Abstract: Recent HST observations have revealed that compact sources exist at the centers of many galaxies across the Hubble sequence. These sources are called ``nuclear star clusters'' (NCs), because their structural properties and scaling relationships are similar to those of globular clusters (GCs). It has been also found that the relationship between the masses of NCs and that of the host galaxies is similar to that obeyed by supermassive black holes (SBHs). In this observational frame, the hypothesis that galactic nuclei may be the remains of GCs driven inward to the galactic center by dynamical friction and there merged, finds an exciting possible confirm. In this short paper we report of our recent results on GC mergers obtained by mean of detailed N-body simulations.

Abstract: The ACS Virgo Cluster Survey by Cￜtￜ and collaborators shows the presence of compact nuclei at the photocenters of many early-type galaxies. It is argued that they are the low mass counterparts of nuclei hosting super-massive black holes detected in bright galaxies. If this view is correct, then one should think in terms of central massive objects, either super-massive black holes or compact stellar clusters that accompany the formation of almost all early-type galaxies. In this observational frame, the hypothesis that galactic nuclei may be the remains of globular clusters driven inward to the galactic center by dynamical friction and there merged, finds an exciting possible confirmation. In this short paper we report on our recent results on globular cluster mergers obtained by means of detailed N-body simulations.

Abstract: We present the results of detailed N-body simulations of clusters moving in a realistic Milky Way potential. The strong interaction with the bulge and the disk of the Galaxy leads to the formation of tidal tails, emanating from opposite sides of the cluster. Their orientation and morphology may be interpreted easily in terms of a comoving frame of coordinates.

Abstract: Numerical simulations of a globular cluster orbiting in the central region of a triaxial galaxy have been performed, in order to study the formation and subsequent evolution of tidal tails and their main features. Tails begin to form after about a quarter of the cluster orbital period and tend to lie along its orbit, with a leading tail that precedes the cluster and an outer tail that trails behind it. Tails show clumpy substructures; the most prominent ones (for a globular cluster moving on a quasi-circular orbit around the galaxy) are located at a distance from the cluster center between 50 pc and 80 pc and, after 3 orbital periods, contain about 10% of the cluster mass at that epoch. The morphology of tails and clumps will be compared with available observational data, in particular with that concerning Palomar 5, for which evident clumps in the tails have been detected. Kinematical properties of stars in the tails (line-of-sight velocities and velocity dispersion profiles) will be presented and compared to kinematical data of M15 and omega Centauri, two galactic globular clusters for which there is evidence that the velocity dispersion remains constant at large radii. All the simulations have been performed with our own implementation of a tree-code, that uses a multipolar expansion of the potential truncated at the quadrupole moment and that ran on high performance computers employing an original parallelization approach implemented via MPI routines. The time-integration of the `particles' trajectories is performed by a 2<SUP>nd</SUP> order leap-frog algorithm, using individual and variable time-steps. Part of this work has been done using the IBM SP4 platform located at CINECA (Bologna, Italy) thanks to the grant inarm007 obtained in the framework of INAF-CINECA agreements.

Abstract: Not Available

Abstract: Astrocomp is a project based on a collaboration among the University of Roma La Sapienza, the Astrophysical Observatory of Catania and ENEA. The main motivation of the AstroComp project is to construct a portal, which allows to set up a repository of computational codes and common databases, making them available and enjoyable, with a user-friendly graphical web interface, to the international community. AstroComp will allow the scientific community to benefit by the use of many different numerical tools implemented on high performance computing (HPC) resources, both for theoretical astrophysics and cosmology and for the storage and analysis of astronomical data, without the need of specific training, know-how and experience either in computational techniques or in database construction and management methods. An essential feature of Astrocomp is that it makes available to subscribers some CPU time on large parallel platforms, via specific grants. Astrocomp is partly financed by a grant of the Italian national research Council (CNR).

Abstract: We followed numerically the dynamics of a self-gravitating system of the size of a typical globular cluster, composed by few hundred thousand stars, starting from non-equilibrium initial conditions. Non-equilibrium initial conditions are expected after the complete exhaustion of primordial gas through the gas-star phase transition which occurred on the star-formation time scale. The results allow to draw some conclusions about the evolution of the distribution function toward a meta-stable state, as due to the quick initial collapse and re-expansion of the whole system. This ``virialized'' state has the, well known, further evolution due to star-star encounters. The initial ``violent'' dynamics can be worthly investigated by mean of codes (like our own) which are efficient in the representation of the time evolution driven by the quick mean-field variations, even if the individual body-body close interactions (not relevant in this phase) are not followed in detail.

Abstract: In this talk we discuss some of the main theoretical problems in the understanding of the statistical properties of gravity. By means of N-body simulations we approach the problem of understanding the rï¿œle of gravity in the clustering of a finite set of N-interacting particles which samples a portion of an infinite system. Through the use of the conditional average density, we study the evolution of the clustering for the system putting in evidence some interesting and not yet understood features of the process. .

Abstract: Not Available

Abstract: The decaying of globular clusters towards galactic nuclei can be an efficient dynamical mechanism to concentrate high amounts of stellar matter in the very inner galactic regions, so to contribute significantly to the accretion and feeding of a central massive black hole. Such decaying is made possible by the dynamical friction which dissipates the cluster orbital kinetic energy in a reasonably short time. Quantitative indications that this mechanism is capable to sustain the observed AGN luminosities have been already given. However, there is need of a more refined numerical approach. In particular, while in normal conditions dynamical friction is well understood and its effects sufficiently well described, it is not clear what happens when a cluster decays into a region which ``encloses'' a bulge mass comparable with that of the cluster itself. In this case the gravitational feed-back of the cluster on the bulge is very important and cannot be neglected. Moreover, it is quite difficult to predict, by just analytical means, the tidal effects due to the presence of the massive black hole on clusters' dynamics. We want to show the results obtained by our simulations in this context. The simulations have been performed both with a serial and a parallel `tree-code' (on a CRAY T3E), using a leap-frog scheme for the integration of particles' trajectories, with individual and variable time steps. A completely self-consistent particle representation has been used, not only for the globular cluster but also for the nuclear region of the bulge and for the massive black hole.

Abstract: In this work, the features and the performances of a parallelization method for a tree-code are described. Such method consists in a ``dynamic'' decomposition of the computational domain, carried out conjointly by all the processor elements, exploiting the adaptivity of the same hierarchical spatial subdivision in boxes which the tree-code is based on. This ensures a good efficiency in the domain decomposition during the whole system evolution. The method shows also low computational costs, thanks to the fact that it is performed `on the fly' during the construction of the tree logical structure. Moreover, it can be implemented both on SMP computers and on clusters of workstations by means of message passing routines.

Abstract: A super compact object seating in the center of an elliptical galaxy may start a strong gravitational activity due to spherical mass accretion in the form of dynamically braked globular clusters as well as in the form of stellar debris of tidally destroyed clusters. The resulting AGN luminosity and life time depend mainly on the black hole size and initial characteristics of the globular cluster family.

Abstract: A modern approach to the evolution of a mixed (stars and gas) self-gravitating system is the fully Lagrangian particle approach. The gaseous (particle) phase differs from the compact because the mutual force is given by the sum of gravity and pressure gradient. In this note we report of some characteristics, advantages and limitations of this approach for what regards the evaluation of forces in the system. In particular, a comparison between classic tree-code and fast multipole algorithm to evaluate gravitational forces is discussed.

Abstract: The ACS Virgo cluster survey by Cote' and collaborators shows the presence of compact nuclei at the photocenters of many early-type galaxies. It is argued that they are the low-mass counterparts of nuclei hosting Super Massive Black Holes (SBHs) detected in the bright galaxies. If this view is correct, then one should think in terms of central massive objects, either SBHs or Compact Stellar Clusters (CSCs), that accompany the formation of almost all early-type galaxies. In this observational frame, the hypothesis that galactic nuclei may be the remains of globular clusters driven inward to the galactic center by dynamical friction and there merged, finds an exciting possible confirm. In this short paper we report of our recent results on globular cluster mergers obtained by mean of detailed N-body simulations.

Abstract: We present the results of detailed N-body simulations regarding the interaction of four massive globular clusters in the central region of a triaxial galaxy. The systems undergo a full merging event, producing a sort of 'Super Star Cluster' (SSC) whose features are close to those of a superposition of the individual initial mergers. In contrast with other similar simulations, the resulting SSC structural parameters are located along the observed scaling relations of globular clusters. These findings seem to support the idea that a massive SSC may have formed in early phases of the mother galaxy evolution and contributed to the growth of a massive nucleus.

Abstract: In the last decade, observational studies have shown the existence of tidal streams in the outer part of many galactic globular clusters. The most striking examples of clusters with well defined tidal tails are represented by Palomar 5 and NGC 5466 (both observed in the framework of the Sloan Digital Sky Survey), which show structures elongated for 4 kpc and 1 kpc in length, respectively. Unfortunately, most of the observational studies about globular clusters (GCs) do not cover such a large field of the sky as the SDSS does. In this framework, by mean of a parallel, adaptive tree-code, we performed detailed N-body simulations of GCs moving in a realistic three-components (bulge, disk and halo) Milky Way potential, in order to clarify whether and to what extent tails in the clusters outer regions (few tidal radii) are tracers of the local orbits and, also, if some kind of correlation exists among the cluster orbital phase and the orientation of such streams.

Abstract: We have preliminary results on the parallelization of a Tree-Code for evaluating gravitational forces in N-body astrophysical systems. For our Cray T3D/CRAFT implementation, we have obtained an encouraging speed-up behavior, which reaches a value of 37 with 64 processor elements (PEs). According to the Amdahl'law, this means that about 99% of the code is actually parallelized. The speed-up tests regarded the evaluation of the forces among N = 130,369 particles distributed scaling the actual distribution of a sample of galaxies seen in the Northern sky hemisphere. Parallelization of the time integration of the trajectories, which has not yet been taken into account, is both easier to implement and not as fundamental.