1,721,005 research outputs found
Globular Clusters and Galactic Nuclei
Full text linkDynamical evolution plays a key role in shaping the current properties of star clusters and star cluster systems. We present the study of stellar dynamics both from a theoretical and numerical point of view. In particular we investigate this topic on different astrophysical scales, from the study of the orbital evolution and the mutual interaction of GCs in the Galactic central region to the evolution of GCs in the larger scale galactic potential.
Globular Clusters (GCs), very old and massive star clusters, are ideal objects to explore many aspects of stellar dynamics and to investigate the dynamical and evolutionary mechanisms of their host galaxy. Almost every surveyed galaxy of sufficiently large mass has an associated group of GCs, i.e. a Globular Cluster System (GCS). The first part of this Thesis is devoted to the study of the evolution of GCSs in elliptical galaxies. Basing on the hypothesis that the GCS and stellar halo in a galaxy were born at the same time and, so, with the same density distribution, a logical consequence is that the presently observed difference may be due to evolution of the GCS. Actually, in this scenario, GCSs evolve due to various mechanisms, among which dynamical friction and tidal interaction with the galactic field are the most important. On the other side, the collisionless stellar halo component stands unchanged, thus the difference between the two profiles may correspond to mass lost by the GCS to the galactic center. There the GCs merge and they contribute to the formation/accretion of a luminous and compact central Nuclear Star Cluster (NSC). This is known as the “merger model” for the formation of NSCs, observed at the center of many galaxies and also in the Milky Way (MW) center.
In the second part of this work a new high performance code, NBSymple, is presented. NBSymple is an efficient N-body integrator implemented on a hybrid CPU+GPU platform, exploiting a double-parallelization on CPUs and on the hosted Graphic Processing Units (GPUs). The precision is guaranteed by direct summa- tion for force evaluation, and on the use of high order, symplectic time integration methods. The code allows the choice between two different symplectic integrators: a second-order algorithm (commonly known as leapfrog) and a much more accurate (but also time consuming) sixth-order method. The effect of the external galactic field is represented as an analytical approximation of its gravitational potential. The code has been widely tested and benchmarked. Moreover, it has been used for various applications (globular clusters quasi-radial orbit through a galactic massive central object, primordial evolution of young stellar clusters, etc.).
NBSymple and another, publicly available, direct summation code, φGRAPE, have been used to explore the the previously described merger mode for the Galactic NSC formation. In particular, we used self consistent N-body simulations where the Galaxy was modeled using observational data about the Milky Way, and including the presence of the Galactic central supermassive black hole. We let decay 12 GCs initially located on different circular orbits at the same galactocentric distance. The merging of clusters in the central zone of the Galaxy and its following evolution due to two-body relaxation generates a NSC that actually resembles the one observed at the center of the MW.
By mean of numerical simulations carried on with NBSymple, we investigated more in detail the dynamical evolution of GCs in the MW potential with particular attention to the formation of clumpy structures in the tidal tails that arise around the orbiting cluster. Although various hypothesis have been proposed, the formation process of these clumps is not yet clearly understood. Through a statistical analysis of the orbital properties of the stars that “escape” from the cluster we aimed to a better understanding of the on going process. Studying and comparing such simulations with observational data we could gain to a deeper knowledge of the shape Galactic potential and, more generally, of the Galactic dynamics.PhD Thesi
EVOLUTION OF SECOND-GENERATION STARS IN STELLAR DISKS OF GLOBULAR AND NUCLEAR CLUSTERS: ω CENTAURI AS A TEST CASE
No full textGlobular clusters (GCs) and many nuclear clusters (NCs) show evidence of hosting multiple generations of stellar populations. Younger stellar populations in NCs appear to reside in disk-like structures, including the NC in our own Galactic center as well as in M31. Kinematic studies of the anomalous GC ω Centauri, thought to possibly be a former dwarf galaxy (or a galactic nucleus), show evidence of hosting a central, kinematically cold disk component. These observations suggest that formation of second- (or multiple) generation stars may occur in flattened disk-like structures. Here, we use detailed N-body simulations to explore the possible evolution of such stellar disks embedded in GCs. We follow the long-term evolution of a disk-like structure similar to that observed in ω Centauri and study its properties. We find that a stellar-disk-like origin for second-generation stellar populations can leave behind significant kinematic signatures in properties of the clusters, including an anisotropic distribution and lower velocity dispersions, which can be used to constrain the origin of second-generations stars and their dynamical evolution. © 2013. The American Astronomical Society. All rights reserved
Globular cluster system erosion in elliptical galaxies
No full textContext. We analyse data of 8 elliptical galaxies to study the difference between the radial distributions of their globular cluster systems (GCSs) and their galactic stellar component. In all galaxies studied, the GCS density profile is significantly flatter towards the galactic centre than that of the stars. Aims. A flatter profile of the radial distribution of the GCS with respect to that of the galactic stellar component is a difference with astrophysical relevance. A quantitative comparative analysis of the profiles may provide insight into both galaxy and globular cluster formation and evolution. If the difference is caused by erosion of the GCS, the missing GCs in the galactic central region may have merged around the galactic centre and formed, or at least increased in mass, the galactic nucleus. Observational support to this are the correlations between the galaxy integrated absolute magnitude and the number of globular clusters lost and that between the central massive black hole mass and the total mass of globular clusters lost. Methods. We fitted both the stellar and globular cluster system radial profiles of a set of galaxies observed at high resolution. We found that the GCS profile is less sharply peaked at the galactic centre than the stellar one. Assuming that this difference is caused by GCS evolution, starting from a radial distribution initially indistinguishable from that of stars, we can evaluate by a simple normalization procedure the number (and mass) of GCs that have "disappeared". Results. The 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 (for every galaxy of the sample) in the 2.77 x 10(7)-1.58 x 10(9) M(circle dot) interval. All the transported mass towards the central galactic regions have had probably an important effect on the innermost galactic zone, including its violent transient activity (AGN) and local massive black hole formation and growth
NBSymple, a double parallel, symplectic N-body code running on graphic processing units
No full textWe present and discuss the characteristics and performance, both in term of computational speed and precision, of a numerical code which integrates the equation of motions of N 'particles' interacting via Newtonian gravitation and move in an external galactic smooth field. The force evaluation on every particle is done by mean of direct summation of the contribution of all the other system's particles, avoiding truncation error. The time integration is done with second-order and sixth-order symplectic schemes. The code, NBSymple, has been parallelized twice, by mean of the Compute Unified Device Architecture (CUDA) to make the all-pair force evaluation as fast as possible on high-performance Graphic Processing Units NVIDIA TESLA C1060, while the O(N) computations are distributed on various CPUs by mean of OpenMP Application Program. The code works both in single-precision floating point arithmetics or in double precision. The use of single-precision allows the use of the GPU performance at best but, of course, limits the precision of simulation in some critical situations. We find a good compromise in using a software reconstruction of double-precision for those variables that are most critical for the overall precision of the code. The code is available on the web site astrowww.phys.uniroma1.it/dolcetta/nbsymple.html. © 2010 Elsevier B.V. All rights reserved
Globular Cluster Clumpy Tidal Tails
No full textTidal tails are interesting structure recently observed around many Globular Clusters. The understanding of the formation of such debris can help to analyse the interplay between internal and external processes in the cluster as well as to probe the potential of the host galaxy where the cluster is orbiting. We report preliminary results of an on-going study on tidal tails by mean of high precision N-body simulations. We focused on tail
substructure (clumps) origin and evolution, which have been observed in the Pal5 extended tails
Dissipationless Formation and Evolution of the Milky Way Nuclear Star Cluster
No full textIn 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 (SMBHs); this is the case, for instance, in the Milky Way. In this paper, we investigate how the presence of an SMBH at the center of the Milky Way 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 direct 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. These clusters, decayed orbitally in the central region due to their large mass, were followed in their inspiral events; as a result, the total accumulated mass by approximate to 10 clusters is about 1.5 x 10(7)M(circle dot). Each cluster is disrupted by the SMBH at a distance of roughly 1 pc. 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 rho similar to r(-2). These properties are similar to those of the Milky Way NSC, with the exception of the core size, which in the Milky Way is somewhat 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 Milky Way) of 10 Gyr as the stellar distribution evolves toward a Bahcall-Wolf cusp. We also show that the luminosity function of the Milky Way NSC is consistent with the hypothesis that 1/2 of the mass comes from old (similar to 10 Gyr) stars, brought in by globular clusters, with the other half due to continuous star formation. We conclude that a model in which a large fraction of the mass of the Milky Way NSC is due to infalling globular clusters is consistent with existing observational constraints
High Performance Astrophysics Computing
No full textThe application of high end computing to astrophysical problems, mainly in the galactic environment, is developing for many years at the Dep. of Physics of Sapienza Univ. of Roma. The main scientific topic is the physics of self gravitating systems, whose specific subtopics are: i) celestial mechanics and interplanetary probe transfers in the solar system; ii) dynamics of globular clusters and of globular cluster systems in their parent galaxies; iii) nuclear clusters formation and evolution; iv) massive black hole formation and evolution; v) young star cluster early evolution. In this poster we describe the software and hardware computational resources available in our group and how we are developing both software and hardware to reach the scientific aims above itemized
Tracing the Evolution of Globular Clusters Through their Tidal Tails
Full text linkopenGalactic globular clusters evolve within the Milky Way’s gravitational potential. As they orbit the Galaxy, these clusters undergo internal evolution while simultaneously losing stars due to the Galactic tidal field. These escaped stars form extended structures known as tidal tails, which typically consist of two stellar streams: one leading the cluster and one trailing it along its orbit. The stars in these tails have been observed in a few clusters and will be studied in greater detail with future surveys, such as WEAVE, 4MOST, and LSST.
This study aims to model the evolution of globular clusters in the presence of multiple stellar populations, using the stars lost at different times to investigate the origin of the anomalous stars found in these systems. To address this, we use high-resolution N-body simulations of a cluster model orbiting in a Milky Way-like potential. To explore the impact of orbital shape on the dynamical evolution of the system, the same cluster is evolved along three different orbital configurations with distinct eccentricities. The simulations cover a total timescale of 12 Gyr and incorporate simplified prescriptions for stellar evolution-driven mass loss, as well as initial conditions based on King models to represent the internal structure of the stellar populations.
Particular attention is given to the formation and structure of the tidal tails, their asymmetries, and the population gradients within them. By analyzing the spatial and kinematic distributions of escaped stars, we explore whether differences between first-generation and second-generation stars persist in the tails and how these relate to the cluster's internal segregation and orbital phase. In this way, tidal tails emerge as powerful tools not only to trace the dynamical history of globular clusters but also to provide clues about the formation and evolution of their multiple stellar populations
A primordial origin for the composition similarity between the Earth and the Moon
No full textInternational audienceMost of the properties of the Earth-Moon system can be explained by a collision between a planetary embryo and the growing Earth late in the accretion process. Simulations show that most of the material that eventually aggregates to form the Moon originates from the impactor. However, analysis of the terrestrial and lunar isotopic composition show them to be highly similar. In contrast, the compositions of other solar system bodies are significantly different than the Earth and Moon. This poses a major challenge to the giant impact scenario since the Moon-forming impactor is then thought to also have differed in composition from the proto-Earth. Here we track the feeding zones of growing planets in a suite of simulations of planetary accretion, in order to measure the composition of Moon-forming impactors. We find that different planets formed in the same simulation have distinct compositions, but the compositions of giant impactors are systematically more similar to the planets they impact. A significant fraction of planet-impactor pairs have virtually identical compositions. Thus, the similarity in composition between the Earth and Moon could be a natural consequence of a late giant impact
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