arXiv subjects

P. Miocchi

Publications and source records attributed to P. Miocchi.

At least 37 records · Page 2Linked to original sources

arXiv2009

Simulations of Nuclear Cluster formation

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=1.5x10^6 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 about 2 times faster than that predicted by the generalized Chandrasekhar formula; ii) the progenitor clusters merge in less than 20 galactic core-crossing time; iii) the NC configuration keeps a quasi-stable state at least within 70 galactic core-crossing times.

astro-ph.CO↗

arXiv2009

Galactic Nuclei Formation Via Globular Cluster Merging

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=1.5x10^6 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 about 2 times faster than that predicted by the generalized Chandrasekhar formula; ii) the progenitor clusters merge in less than 20 galactic core-crossing times; iii) the NC configuration keeps quasi-stable at least within 70 galactic core-crossing times.

astro-ph.GA↗

arXiv2008

Self-consistent simulations of Nuclear Cluster formation through Globular Cluster orbital decay and merging

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.

astro-ph↗

arXiv2008

Merging of globular clusters within inner galactic regions. II. The Nuclear Star Cluster formation

In this paper we present the results of two detailed N-body simulations of the interaction of a sample of four massive globular clusters in the inner region of a triaxial galaxy. A full merging of the clusters takes place, leading to a slowly evolving cluster which is quite similar to observed Nuclear Clusters. Actually, both the density and the velocity dispersion profiles match qualitatively, and quantitatively after scaling, with 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 central progenitors central densities. These findings support the idea that a massive Nuclear Cluster may have formed in early phases of the mother galaxy evolution and lead to the formation of a nucleus, which, in many galaxies, has indeed a luminosity profile similar to that of an extended King model. A correlation with galactic nuclear activity is suggested.

astro-ph↗

arXiv2007

On the Origin of Early-type Galaxies Nuclei

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.

astro-ph↗

arXiv2007

Tidal tails around globular clusters

astro-ph↗

arXiv2007

The surface density profile of NGC 6388: a good candidate for harboring an intermediate-mass black hole

We have used a combination of high resolution (HST ACS-HRC, ACS-WFC, and WFPC2) and wide-field (ESO-WFI) 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 intermediate concentration (c=1.8) and sizable core radius (rc=7"), a significant power law (with slope α=-0.2) deviation from a flat core behavior has been detected within the inner 1 arcsecond. These properties suggest the presence of a central intermediate mass black hole. The observed profiles are well reproduced by a multi-mass isotropic, spherical model including a black hole with a mass of ~5.7x10^3 Msol.

astro-ph↗

arXiv2007

The presence of intermediate-mass black holes in globular clusters and their connection with extreme horizontal branch stars

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_BH) 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_BH < M/100), and c decreases monotonically with M_BH. 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_BH > M. The mass range estimate 12s - 4.8 < log (M_BH/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_BH ~ 0.09(M/M_sol)^{0.7} M_sol and M_BH ~ 50[sigma/ (km s^-1)]^{1.2} M_sol 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.

astro-ph↗

arXiv2007

Tidal tails around globular clusters. Are they a good tracer of cluster orbits?

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 intepreted. The tails have a complex morphology, in particular 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 to the cluster orbital phase and to 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.

astro-ph↗

arXiv2006

Tidal tails around globular clusters: are they good tracers of cluster orbits?

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.

astro-ph↗

arXiv2006

Simulations of globular clusters merging in galactic nuclear regions

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.

astro-ph↗

arXiv2006

Merging of globular clusters within inner galactic regions. I. Do they survive the tidal interaction?

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 (order of 10^7 solar masses) GCs that were able to decay, because of dynamical friction, into the inner regions of triaxial galaxies on a time much shorter than their internal relaxation time. To check the disruptive role of both tidal forces and GC-GC collisions, we maximised the tidal interaction considering GCs on quasi-radial orbits. The available CPU resources allowed us to follow 8 oscillations of the GCs along their orbits and the main findings are: i) clusters with an initial high enough King concentration parameter (c>=1.2), preserve up to 50% of their initial mass; ii) the inner density distribution of the survived clusters keep a King model profile; iii) GC-GC collisions have a negligible effect with respect to that caused by the passage through the galactic center; iv) the orbital energy dissipation due to the tidal interaction is of the same order of that caused by dynamical friction; v) complex sub-structures like "ripples" and "clumps" 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 investigations.

astro-ph↗

arXiv2005

Central energy equipartition in multi-mass models of globular clusters

In the construction of multi-mass King-Michie models of globular clusters, an approximated central energy equipartition between stars of different masses 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' (IA) 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 IA, 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 3-component model fitting a given projected surface brightness and line-of-sight velocity dispersion profiles is discussed. In this example, the IA gives a cluster envelope much more concentrated (central dimensionless potential W=3.3) than under the true equipartition (W=0.059), as well as a higher logarithmic mass function slope. As a consequence, the inferred total mass (and then the global mass-to-light ratio) results a factor 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 giants and turn-off stars component.

astro-ph↗

arXiv2004

Astrocomp: a web service for the use of high performance computers in Astrophysics

Astrocomp is a joint project, developed by the INAF-Astrophysical Observatory of Catania, University of Roma La Sapienza and Enea. The project has the goal of providing the scientific community of a web-based user-friendly interface which allows running parallel codes on a set of high-performance computing (HPC) resources, without any need for specific knowledge about parallel programming and Operating Systems commands. Astrocomp provides, also, computing time on a set of parallel computing systems, available to the authorized user. At present, the portal makes a few codes available, among which: FLY, a cosmological code for studying three-dimensional collisionless self-gravitating systems with periodic boundary conditions; ATD, a parallel tree-code for the simulation of the dynamics of boundary-free collisional and collisionless self-gravitating systems and MARA, a code for stellar light curves analysis. Other codes are going to be added to the portal.

astro-ph↗

arXiv2004

Formation and evolution of clumpy tidal tails around globular clusters

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 7,000 solar masses 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 to that found for M15 and Omega Centauri.

astro-ph↗

arXiv2003

Super computers in astrophysics and High Performance simulations of self-gravitating systems

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.

astro-ph↗

arXiv2001

Gravitational clustering in N-body simulations

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 rôle 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.

cond-mat.stat-mech↗

arXiv2001

Clustering in gravitating N-body systems

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 cluste ring. 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.

cond-mat.stat-mech↗