20 research outputs found
Hydra: A Parallel Adaptive Grid Code
We describe the first parallel implementation of an adaptive
particle-particle, particle-mesh code with smoothed particle hydrodynamics.
Parallelisation of the serial code, ``Hydra'', is achieved by using CRAFT, a
Cray proprietary language which allows rapid implementation of a serial code on
a parallel machine by allowing global addressing of distributed memory.
The collisionless variant of the code has already completed several 16.8
million particle cosmological simulations on a 128 processor Cray T3D whilst
the full hydrodynamic code has completed several 4.2 million particle combined
gas and dark matter runs. The efficiency of the code now allows parameter-space
explorations to be performed routinely using particles of each species.
A complete run including gas cooling, from high redshift to the present epoch
requires approximately 10 hours on 64 processors.
In this paper we present implementation details and results of the
performance and scalability of the CRAFT version of Hydra under varying degrees
of particle clustering.Comment: 23 pages, LaTex plus encapsulated figure
A Parallel Adaptive P3M code with Hierarchical Particle Reordering
We discuss the design and implementation of HYDRA_OMP a parallel
implementation of the Smoothed Particle Hydrodynamics-Adaptive P3M (SPH-AP3M)
code HYDRA. The code is designed primarily for conducting cosmological
hydrodynamic simulations and is written in Fortran77+OpenMP. A number of
optimizations for RISC processors and SMP-NUMA architectures have been
implemented, the most important optimization being hierarchical reordering of
particles within chaining cells, which greatly improves data locality thereby
removing the cache misses typically associated with linked lists. Parallel
scaling is good, with a minimum parallel scaling of 73% achieved on 32 nodes
for a variety of modern SMP architectures. We give performance data in terms of
the number of particle updates per second, which is a more useful performance
metric than raw MFlops. A basic version of the code will be made available to
the community in the near future.Comment: 34 pages, 12 figures, accepted for publication in Computer Physics
Communication
Simulation techniques for cosmological simulations
Modern cosmological observations allow us to study in great detail the
evolution and history of the large scale structure hierarchy. The fundamental
problem of accurate constraints on the cosmological parameters, within a given
cosmological model, requires precise modelling of the observed structure. In
this paper we briefly review the current most effective techniques of large
scale structure simulations, emphasising both their advantages and
shortcomings. Starting with basics of the direct N-body simulations appropriate
to modelling cold dark matter evolution, we then discuss the direct-sum
technique GRAPE, particle-mesh (PM) and hybrid methods, combining the PM and
the tree algorithms. Simulations of baryonic matter in the Universe often use
hydrodynamic codes based on both particle methods that discretise mass, and
grid-based methods. We briefly describe Eulerian grid methods, and also some
variants of Lagrangian smoothed particle hydrodynamics (SPH) methods.Comment: 42 pages, 16 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 12; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
Reionization by active sources and its effects on the cosmic microwave background
We investigate the possible effects of reionization by active sources on the
cosmic microwave background. We concentrate on the sources themselves as the
origin of reionization, rather than early object formation, introducing an
extra period of heating motivated by the active character of the perturbations.
Using reasonable parameters, this leads to four possibilities depending on the
time and duration of the energy input: delayed last scattering, double last
scattering, shifted last scattering and total reionization. We show that these
possibilities are only very weakly constrained by the limits on spectral
distortions from the COBE FIRAS measurements. We illustrate the effects of
these reionization possibilities on the angular power spectrum of temperature
anisotropies and polarization for simple passive isocurvature models and simple
coherent sources, observing the difference between passive and active models.
Finally, we comment on the implications of this work for more realistic active
sources, such as causal white noise and topological defect models. We show for
these models that non-standard ionization histories can shift the peak in the
CMB power to larger angular scales.Comment: 21 pages LaTeX with 11 eps figures; replaced with final version
accepted for publication in Phys. Rev.
Pregalactic activity Some consequences for galaxy formation
SIGLEAvailable from British Library Document Supply Centre- DSC:D59097 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
The effect of radiative cooling on the X-ray properties of galaxy clusters
In this paper, we investigate the effect of cooling on the X-ray properties
of galaxy clusters. We have performed N-body, hydrodynamical simulations both
with and without the effects of radiative cooling, but neglecting the effects
of star formation and feedback. We show that radiative cooling produces an
inflow of high-entropy gas from the outer parts of the cluster, thus
\emph{raising} the cluster temperature and \emph{decreasing} the X-ray
luminosity. With radiative cooling clusters are on average three to five times
less luminous in X-rays than the same cluster simulated without cooling.
However, we do not produce a large constant-density core in either the gas or
the dark matter distributions. Our results contradict previous work in which
cooling raises the X-ray luminosity and deposits an unreasonably large amount
of mass in the central cluster galaxy. We achieve this by selecting our
numerical resolution in such a way that a reasonable fraction of the baryonic
material cools and by decoupling the hot and cold gas in our simulations, a
first step towards modelling multiphase gas. We emphasise that globally cooling
a sensible amount of material is vital and the presence or absence of massive
central concentrations of cold baryonic material has a dramatic effect upon the
resultant X-ray properties of the clusters.Comment: 14 pages, accepted to MNRAS, substantially revised from previous
versio
The effect of Radiative Cooling on X-ray Emission from Clusters of Galaxies
In this paper we use state-of-the-art N-body hydrodynamic simulations of a
cosmological volume of side 100Mpc to produce many galaxy clusters
simultaneously in both the standard cold dark matter (SCDM) cosmology and a
cosmology with a positive cosmological constant (LCDM). We have performed
simulations of the same volume both with and without the effects of radiative
cooling, but in all cases neglect the effects of star formation and feedback.
With radiative cooling clusters are on average five times less luminous in
X-rays than the same cluster simulated without cooling. The importance of the
mass of the central galaxy in determining the X-ray luminosity is stressed.Comment: 4 pages, Conference proceedings - LSS in the X-ray Universe -
Santorin
A simulated ÏCDM cosmology cluster catalogue: the NFW profile and the temperature-mass scaling relations
We have extracted over 400 clusters, covering more than two decades in mass, from three simulations of the ÏCDM cosmology. This represents the largest uniform catalogue of simulated clusters ever produced. The clusters exhibit a wide variety of density profiles. Only a minority are well-fitted in their outer regions by the widely used density profile of Navarro, Frenk & White (NFW), which is applicable to relaxed haloes. Others have steeper outer density profiles, show sharp breaks in their density profiles, or have significant substructure. If we force a fit to the NFW profile, then the best-fitting concentrations decline with increasing mass, but this is driven primarily by an increase in substructure as one moves to higher masses. The temperatureâmass relations for properties measured within a sphere enclosing a fixed overdensity all follow the self-similar form, TâM2/3; however, the normalization is lower than the value inferred for observed clusters. The temperatureâmass relations for properties measured within a fixed physical radius are significantly steeper then this. Both can be accurately predicted using the NFW model
The mass function of dark matter haloes
We combine data from a number of N-body simulations to predict the abundance of dark haloes in cold dark matter (CDM) universes over more than four orders of magnitude in mass. A comparison of different simulations suggests that the dominant uncertainty in our results is systematic and is smaller than 10â30 per cent at all masses, depending on the halo definition used. In particular, our 'Hubble volume' simulations of ÏCDM and ÎCDM cosmologies allow the abundance of massive clusters to be predicted with uncertainties well below those expected in all currently planned observational surveys. We show that for a range of CDM cosmologies and for a suitable halo definition, the simulated mass function is almost independent of epoch, of cosmological parameters and of the initial power spectrum when expressed in appropriate variables. This universality is of exactly the kind predicted by the familiar PressâSchechter model, although this model predicts a mass function shape that differs from our numerical results, overestimating the abundance of 'typical' haloes and underestimating that of massive systems
Simulations of deep pencil-beam redshift surveys
We create mock pencil-beam redshift surveys from very large cosmological
-body simulations of two Cold Dark Matter cosmogonies, an Einstein-de Sitter
model (CDM) and a flat model with and a cosmological
constant (CDM). We use these to assess the significance of the
apparent periodicity discovered by Broadhurst et al. (1990). Simulation
particles are tagged as `galaxies' so as to reproduce observed present-day
correlations. They are then identified along the past light-cones of
hypothetical observers to create mock catalogues with the geometry and the
distance distribution of the Broadhurst et al. data. We produce 1936 (2625)
quasi-independent catalogues from our CDM (CDM) simulation. A
couple of large clumps in a catalogue can produce a high peak at low
wavenumbers in the corresponding one-dimensional power spectrum, without any
apparent large-scale periodicity in the original redshift histogram. Although
the simulated redshift histograms frequently display regularly spaced clumps,
the spacing of these clumps varies between catalogues and there is no
`preferred' period over our many realisations. We find only a 0.72 (0.49) per
cent chance that the highest peak in the power spectrum of a CDM
(CDM) catalogue has a peak-to-noise ratio higher than that in the
Broadhurst et al. data. None of the simulated catalogues with such high peaks
shows coherently spaced clumps with a significance as high as that of the real
data. We conclude that in CDM universes, the kind of regularity observed by
Broadhurst et al. has a priori probability well below .Comment: 8 pages, 9 figures, submitted to MNRA