290 research outputs found
High performance astrophysics computing
The application of high end computing to astrophysical problems, mainly in
the galactic environment, is under development since 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.Comment: 2 pages paper presented at the Conference "Advances in Computational
Astrophysics: methods, tools and outcomes", to be published in the ASP
Conference Series, 2012, vol. 453, R. Capuzzo-Dolcetta, M. Limongi and A.
Tornambe' ed
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.Comment: 18 pages, 10 figures, 3 tables. Submitted to ApJ, main journa
A fully parallel, high precision, N-body code running on hybrid computing platforms
We present a new implementation of the numerical integration of the
classical, gravitational, N-body problem based on a high order Hermite's
integration scheme with block time steps, with a direct evaluation of the
particle-particle forces. The main innovation of this code (called HiGPUs) is
its full parallelization, exploiting both OpenMP and MPI in the use of the
multicore Central Processing Units as well as either Compute Unified Device
Architecture (CUDA) or OpenCL for the hosted Graphic Processing Units. We
tested both performance and accuracy of the code using up to 256 GPUs in the
supercomputer IBM iDataPlex DX360M3 Linux Infiniband Cluster provided by the
italian supercomputing consortium CINECA, for values of N up to 8 millions. We
were able to follow the evolution of a system of 8 million bodies for few
crossing times, task previously unreached by direct summation codes. The code
is freely available to the scientific community.Comment: Paper submitted to Journal of Computational Physics consisting in 28
pages, 9 figures.The previous submitted version was lacking of the
bibliography, for a Tex proble
Elliptical galaxy nuclei activity powered by infalling globular clusters
Globular cluster systems evolve, in galaxies, due to internal and external
dynamics and tidal phenomena. One of the causes of evolution, dynamical
friction, is responsible for the orbital decay of massive clusters into the
innermost galactic regions. It is found that these clusters are effective
source of matter to feed a central galactic black hole such to make it grow and
shine as an AGN.Comment: 8 pages, 2 eps figures, in press in the Proc. of the Meeting Baryons
in Cosmic Structures, Monte Porzio (Italy), oct. 20-21 2003, ASP Conf. Ser.,
eds. E. Giallongo, G. De Zotti, N. Menc
Forming the First Stars in the Universe: The Fragmentation of Primordial Gas
In order to constrain the initial mass function (IMF) of the first generation
of stars (Population III), we investigate the fragmentation properties of
metal-free gas in the context of a hierarchical model of structure formation.
We investigate the evolution of an isolated 3-sigma peak of mass 2x10^6 M_solar
which collapses at z_coll=30 using Smoothed Particle Hydrodynamics. We find
that the gas dissipatively settles into a rotationally supported disk which has
a very filamentary morphology. The gas in these filaments is Jeans unstable
with M_J~10^3 M_solar. Fragmentation leads to the formation of high density
(n>10^8 cm^-3) clumps which subsequently grow in mass by accreting surrounding
gas and by merging with other clumps up to masses of ~10^4 M_solar. This
suggests that the very first stars were rather massive. We explore the complex
dynamics of the merging and tidal disruption of these clumps by following their
evolution over a few dynamical times.Comment: 7 pages, 3 figures, uses emulateapj.sty. Accepted for publication in
the Astrophysical Journal Letter
Dynamical non-axisymmetric instabilities in rotating relativistic stars
We present new results on dynamical instabilities in rapidly rotating
neutron-stars. In particular, using numerical simulations in full General
Relativity, we analyse the effects that the stellar compactness has on the
threshold for the onset of the dynamical bar-mode instability, as well as on
the appearance of other dynamical instabilities. By using an extrapolation
technique developed and tested in our previous study [1], we explicitly
determine the threshold for a wide range of compactnesses using four sequences
of models of constant baryonic mass comprising a total of 59 stellar models.
Our calculation of the threshold is in good agreement with the Newtonian
prediction and improves the previous post-Newtonian estimates. In addition, we
find that for stars with sufficiently large mass and compactness, the m=3
deformation is the fastest growing one. For all of the models considered, the
non-axisymmetric instability is suppressed on a dynamical timescale with an m=1
deformation dominating the final stages of the instability. These results,
together with those presented in [1], suggest that an m=1 deformation
represents a general and late-time feature of non-axisymmetric dynamical
instabilities both in full General Relativity and in Newtonian gravity.Comment: To appear on CQG, NFNR special issue. 16 pages, 5 color figures,
movies from http://www.fis.unipr.it/numrel/BarMode/ResearchBarMode.htm
Land use alters trophic redundancy and resource flow through stream food webs.
The changes to physical and chemical ecosystem characteristics as a response to pervasive and intensifying land use have the potential to alter the consumer-resource interactions and to rewire the flow of energy through entire food webs. We investigated these structural and functional properties of food webs in stream ecosystems distributed across woodland, agricultural and urban areas in the Zagreb region of Croatia. We compared resource availability and consumer diet composition using stable isotope mixing models and tested how the isotopic variance of basal resources, primary consumers, macroinvertebrate predators and other food web characteristics change with different land-use types. Combination of increased loading and altered composition of nutrients, lower water discharge and higher light availability at urban sites likely promoted the contribution of aquatic macrophytes to diets of primary consumers. Macroinvertebrate predators shifted their diet, relying more on active filterers at urban sites relative to woodland and agricultural sites. Urban food webs also had lower trophic redundancy (i.e. fewer species at each trophic level) and a more homogenized energy flow from lower to higher trophic levels. There was no effect of land use on isotopic variation of basal resources, primary consumers or macroinvertebrate predators, but all these trophic groups at urban and agricultural sites were 15 N-enriched relative to their counterparts in woodland stream food webs. The physical and chemical ecosystem characteristics associated with intensive land use altered the resource availability, trophic redundancy and the flow of energy to other trophic levels, with potentially negative consequences for community dynamics and ecosystem functioning. These empirical findings indicate that reducing nutrient pollution, agricultural runoffs and maintaining riparian vegetation can mitigate the impacts of land use on structure and function of stream ecosystems
M87, Globular Clusters, and Galactic Winds: Issues in Giant Galaxy Formation
New VRI photometry is presented for the globular clusters in the innermost
140'' of the M87 halo. The results are used to discuss several issues
concerning the formation and evolution of globular cluster systems in
supergiant ellipticals like M87. (1) we find no significant change in the
globular cluster luminosity function (GCLF) with galactocentric radius, for
cluster masses M < 10^5 solar masses, indicating that the main effects of
dynamical evolution may be only on lower-mass clusters. (2) Within the core
radius (1') of the globular cluster system, the metallicity distribution is
uniform, but at larger radii the mean metallicity declines steadily as Z ~
r^-0.9. (3) The various options for explaining the existence of high specific
frequency galaxies like M87 are evaluated, and scaling laws for the GCSs in
these galaxies are given. Interpretations involving secondary evolution
(formation of many globular clusters during mergers, intergalactic globular
clusters, etc.) are unlikely to be the primary explanation for high-S_N
galaxies. (4) We suggest that central-supergiant E galaxies may have formed in
an exceptionally turbulent or high-density environment in which an early,
powerful galactic wind drove out a high fraction of the protogalactic gas, thus
artificially boosting the specificComment: 67 pages, 17 figures. To appear in Astronomical Journal, in press for
May 1998. Preprints also available from W.Harris; send e-mail request to
[email protected]
Interplay Between Chaotic and Regular Motion in a Time-Dependent Barred Galaxy Model
We study the distinction and quantification of chaotic and regular motion in
a time-dependent Hamiltonian barred galaxy model. Recently, a strong
correlation was found between the strength of the bar and the presence of
chaotic motion in this system, as models with relatively strong bars were shown
to exhibit stronger chaotic behavior compared to those having a weaker bar
component. Here, we attempt to further explore this connection by studying the
interplay between chaotic and regular behavior of star orbits when the
parameters of the model evolve in time. This happens for example when one
introduces linear time dependence in the mass parameters of the model to mimic,
in some general sense, the effect of self-consistent interactions of the actual
N-body problem. We thus observe, in this simple time-dependent model also, that
the increase of the bar's mass leads to an increase of the system's chaoticity.
We propose a new way of using the Generalized Alignment Index (GALI) method as
a reliable criterion to estimate the relative fraction of chaotic vs. regular
orbits in such time-dependent potentials, which proves to be much more
efficient than the computation of Lyapunov exponents. In particular, GALI is
able to capture subtle changes in the nature of an orbit (or ensemble of
orbits) even for relatively small time intervals, which makes it ideal for
detecting dynamical transitions in time-dependent systems.Comment: 21 pages, 9 figures (minor typos fixed) to appear in J. Phys. A:
Math. Theo
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