64 research outputs found
GADGET: A code for collisionless and gasdynamical cosmological simulations
We describe the newly written code GADGET which is suitable both for
cosmological simulations of structure formation and for the simulation of
interacting galaxies. GADGET evolves self-gravitating collisionless fluids with
the traditional N-body approach, and a collisional gas by smoothed particle
hydrodynamics. Along with the serial version of the code, we discuss a parallel
version that has been designed to run on massively parallel supercomputers with
distributed memory. While both versions use a tree algorithm to compute
gravitational forces, the serial version of GADGET can optionally employ the
special-purpose hardware GRAPE instead of the tree. Periodic boundary
conditions are supported by means of an Ewald summation technique. The code
uses individual and adaptive timesteps for all particles, and it combines this
with a scheme for dynamic tree updates. Due to its Lagrangian nature, GADGET
thus allows a very large dynamic range to be bridged, both in space and time.
So far, GADGET has been successfully used to run simulations with up to 7.5e7
particles, including cosmological studies of large-scale structure formation,
high-resolution simulations of the formation of clusters of galaxies, as well
as workstation-sized problems of interacting galaxies. In this study, we detail
the numerical algorithms employed, and show various tests of the code. We
publically release both the serial and the massively parallel version of the
code.Comment: 32 pages, 14 figures, replaced to match published version in New
Astronomy. For download of the code, see
http://www.mpa-garching.mpg.de/gadget (new version 1.1 available
Inferring the dark matter power spectrum from the Lyman-alpha forest in high-resolution QSO absorption spectra
We use the LUQAS sample (Kim et al. 2004), a set of 27 high-resolution and
high signal-to-noise QSO absorption spectra at a median redshift of z=2.25, and
the data from Croft et al. (2002) at a median redshift of z=2.72, together with
a large suite of high-resolution large box-size hydro-dynamical simulations, to
estimate the linear dark matter power spectrum on scales 0.003 s/km < k <0.03
s/km. Our re-analysis of the Croft et al. data agrees well with their results
if we assume the same mean optical depth and gas temperature-density relation.
The inferred linear dark matter power spectrum at z=2.72 also agrees with that
inferred from LUQAS at lower redshift if we assume that the increase of the
amplitude is due to gravitational growth between these redshifts. We further
argue that the smaller mean optical depth measured from high-resolution spectra
is more accurate than the larger value obtained from low-resolution spectra by
Press et al. (1993) which Croft et al. used. For the smaller optical depth we
obtain a ~ 20% higher value for the rms fluctuation amplitude of the matter
density. By combining the amplitude of the matter power spectrum inferred from
the Lyman-alpha forest with the amplitude on large scales inferred from
measurements of the CMB we obtain constraints on the primordial spectral index
n and the normalisation sigma_8. For values of the mean optical depth favoured
by high-resolution spectra, the inferred linear power spectrum is consistent
with a LambdaCDM model with a scale-free (n=1) primordial power spectrum.Comment: 13 pages, 9 figures, 6 tables. Very minor changes to match the
accepted version. MNRAS in pres
The impact of Early Dark Energy on non-linear structure formation
We study non-linear structure formation in high-resolution simulations of
Early Dark Energy (EDE) cosmologies and compare their evolution with the
standard LCDM model. Extensions of the spherical top-hat collapse model predict
that the virial overdensity and linear threshold density for collapse should be
modified in EDE model, yielding significant modifications in the expected halo
mass function. Here we present numerical simulations that directly test these
expectations. Interestingly, we find that the Sheth & Tormen formalism for
estimating the abundance of dark matter halos continues to work very well in
its standard form for the Early Dark Energy cosmologies, contrary to analytic
predictions. The residuals are even slightly smaller than for LCDM. We also
study the virial relationship between mass and dark matter velocity dispersion
in different dark energy cosmologies, finding excellent agreement with the
normalization for Lambda as calibrated by Evrard et al.(2008). The earlier
growth of structure in EDE models relative to LCDM produces large differences
in the mass functions at high redshift. This could be measured directly by
counting groups as a function of the line-of-sight velocity dispersion,
skirting the ambiguous problem of assigning a mass to the halo. Using dark
matter substructures as a proxy for member galaxies, we demonstrate that even
with 3-5 members sufficiently accurate measurements of the halo velocity
dispersion function are possible. Finally, we determine the concentration-mass
relationship for our EDE cosmologies. Consistent with the earlier formation
time, the EDE halos show higher concentrations at a given halo mass. We find
that the magnitude of the difference in concentration is well described by the
prescription of Eke et al.(2001) for estimating halo concentrations.Comment: 17 pages,17 figure
The fine-grained phase-space structure of Cold Dark Matter halos
We present a new and completely general technique for calculating the
fine-grained phase-space structure of dark matter throughout the Galactic halo.
Our goal is to understand this structure on the scales relevant for direct and
indirect detection experiments. Our method is based on evaluating the geodesic
deviation equation along the trajectories of individual DM particles. It
requires no assumptions about the symmetry or stationarity of the halo
formation process. In this paper we study general static potentials which
exhibit more complex behaviour than the separable potentials studied
previously. For ellipsoidal logarithmic potentials with a core, phase mixing is
sensitive to the resonance structure, as indicated by the number of independent
orbital frequencies. Regions of chaotic mixing can be identified by the very
rapid decrease in the real space density of the associated dark matter streams.
We also study the evolution of stream density in ellipsoidal NFW halos with
radially varying isopotential shape, showing that if such a model is applied to
the Galactic halo, at least streams are expected near the Sun. The most
novel aspect of our approach is that general non-static systems can be studied
through implementation in a cosmological N-body code. Such an implementation
allows a robust and accurate evaluation of the enhancements in annihilation
radiation due to fine-scale structure such as caustics. We embed the scheme in
the current state-of-the-art code GADGET-3 and present tests which demonstrate
that N-body discreteness effects can be kept under control in realistic
configurations.Comment: 20 pages, 24 figures, submitted to MNRA
Populating a cluster of galaxies - I. Results at z=0
We simulate the assembly of a massive rich cluster and the formation of its
constituent galaxies in a flat, low-density universe. Our most accurate model
follows the collapse, the star-formation history and the orbital motion of all
galaxies more luminous than the Fornax dwarf spheroidal, while dark halo
structure is tracked consistently throughout the cluster for all galaxies more
luminous than the SMC. Within its virial radius this model contains about 2.0e7
dark matter particles and almost 5000 distinct dynamically resolved galaxies.
Simulations of this same cluster at a variety of resolutions allow us to check
explicitly for numerical convergence both of the dark matter structures
produced by our new parallel N-body and substructure identification codes, and
of the galaxy populations produced by the phenomenological models we use to
follow cooling, star formation, feedback and stellar aging. This baryonic
modelling is tuned so that our simulations reproduce the observed properties of
isolated spirals outside clusters. Without further parameter adjustment our
simulations then produce a luminosity function, a mass-to-light ratio,
luminosity, number and velocity dispersion profiles, and a morphology-radius
relation which are similar to those observed in real clusters. In particular,
since our simulations follow galaxy merging explicitly, we can demonstrate that
it accounts quantitatively for the observed cluster population of bulges and
elliptical galaxies.Comment: 28 pages, submitted to MNRA
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