80 research outputs found
Vlasov versus N-body: the H\'enon sphere
We perform a detailed comparison of the phase-space density traced by the
particle distribution in Gadget simulations to the result obtained with a
spherical Vlasov solver using the splitting algorithm. The systems considered
are apodized H\'enon spheres with two values of the virial ratio, R ~ 0.1 and
0.5. After checking that spherical symmetry is well preserved by the N-body
simulations, visual and quantitative comparisons are performed. In particular
we introduce new statistics, correlators and entropic estimators, based on the
likelihood of whether N-body simulations actually trace randomly the Vlasov
phase-space density. When taking into account the limits of both the N-body and
the Vlasov codes, namely collective effects due to the particle shot noise in
the first case and diffusion and possible nonlinear instabilities due to finite
resolution of the phase-space grid in the second case, we find a spectacular
agreement between both methods, even in regions of phase-space where nontrivial
physical instabilities develop. However, in the colder case, R=0.1, it was not
possible to prove actual numerical convergence of the N-body results after a
number of dynamical times, even with N=10 particles.Comment: 19 pages, 11 figures, MNRAS, in pres
The three dimensional skeleton: tracing the filamentary structure of the universe
The skeleton formalism aims at extracting and quantifying the filamentary
structure of the universe is generalized to 3D density fields; a numerical
method for computating a local approximation of the skeleton is presented and
validated here on Gaussian random fields. This method manages to trace well the
filamentary structure in 3D fields such as given by numerical simulations of
the dark matter distribution on large scales and is insensitive to monotonic
biasing. Two of its characteristics, namely its length and differential length,
are analyzed for Gaussian random fields. Its differential length per unit
normalized density contrast scales like the PDF of the underlying density
contrast times the total length times a quadratic Edgeworth correction
involving the square of the spectral parameter. The total length scales like
the inverse square smoothing length, with a scaling factor given by 0.21 (5.28+
n) where n is the power index of the underlying field. This dependency implies
that the total length can be used to constrain the shape of the underlying
power spectrum, hence the cosmology. Possible applications of the skeleton to
galaxy formation and cosmology are discussed. As an illustration, the
orientation of the spin of dark halos and the orientation of the flow near the
skeleton is computed for dark matter simulations. The flow is laminar along the
filaments, while spins of dark halos within 500 kpc of the skeleton are
preferentially orthogonal to the direction of the flow at a level of 25%.Comment: 17 pages, 11 figures, submitted to MNRA
On the Onset of Stochasticity in CDM Cosmological Simulations
The onset of stochasticity is measured in CDM cosmological
simulations using a set of classical observables. It is quantified as the local
derivative of the logarithm of the dispersion of a given observable (within a
set of different simulations differing weakly through their initial
realization), with respect to the cosmic growth factor. In an Eulerian
framework, it is shown here that chaos appears at small scales, where dynamic
is non-linear, while it vanishes at larger scales, allowing the computation of
a critical transition scale corresponding to ~ 3.5 Mpc/h. This picture is
confirmed by Lagrangian measurements which show that the distribution of
substructures within clusters is partially sensitive to initial conditions,
with a critical mass upper bound scaling roughly like the perturbation's
amplitude to the power 0.15. The corresponding characteristic mass, , is roughly of the order of the critical mass of non
linearities at z=1 and accounts for the decoupling induced by the dark energy
triggered acceleration. The sensitivity to detailed initial conditions spills
to some of the overall physical properties of the host halo (spin and velocity
dispersion tensor orientation) while other "global" properties are quite robust
and show no chaos (mass, spin parameter, connexity and center of mass
position). This apparent discrepancy may reflect the fact that quantities which
are integrals over particles rapidly average out details of difference in
orbits, while the other observables are more sensitive to the detailed
environment of forming halos and reflect the non-linear scale coupling
characterizing the environments of halos.Comment: 11 pages, 10 figures. Accepted for publication, MNRA
On the filamentary environment of galaxies
The correlation between the large-scale distribution of galaxies and their
spectroscopic properties at z=1.5 is investigated using the Horizon MareNostrum
cosmological run.
We have extracted a large sample of 10^5 galaxies from this large
hydrodynamical simulation featuring standard galaxy formation physics. Spectral
synthesis is applied to these single stellar populations to generate spectra
and colours for all galaxies. We use the skeleton as a tracer of the cosmic web
and study how our galaxy catalogue depends on the distance to the skeleton. We
show that galaxies closer to the skeleton tend to be redder, but that the
effect is mostly due to the proximity of large haloes at the nodes of the
skeleton, rather than the filaments themselves.
This effects translate into a bimodality in the colour distribution of our
sample. The origin of this bimodality is investigated and seems to follow from
the ram pressure stripping of satellite galaxies within the more massive
clusters of the simulation.
The virtual catalogues (spectroscopical properties of the MareNostrum
galaxies at various redshifts) are available online at
http://www.iap.fr/users/pichon/MareNostrum/cataloguesComment: 18 pages, 27 figures, accepted for publication in MNRA
The persistent cosmic web and its filamentary structure I: Theory and implementation
We present DisPerSE, a novel approach to the coherent multi-scale
identification of all types of astrophysical structures, and in particular the
filaments, in the large scale distribution of matter in the Universe. This
method and corresponding piece of software allows a genuinely scale free and
parameter free identification of the voids, walls, filaments, clusters and
their configuration within the cosmic web, directly from the discrete
distribution of particles in N-body simulations or galaxies in sparse
observational catalogues. To achieve that goal, the method works directly over
the Delaunay tessellation of the discrete sample and uses the DTFE density
computed at each tracer particle; no further sampling, smoothing or processing
of the density field is required.
The idea is based on recent advances in distinct sub-domains of computational
topology, which allows a rigorous application of topological principles to
astrophysical data sets, taking into account uncertainties and Poisson noise.
Practically, the user can define a given persistence level in terms of
robustness with respect to noise (defined as a "number of sigmas") and the
algorithm returns the structures with the corresponding significance as sets of
critical points, lines, surfaces and volumes corresponding to the clusters,
filaments, walls and voids; filaments, connected at cluster nodes, crawling
along the edges of walls bounding the voids. The method is also interesting as
it allows for a robust quantification of the topological properties of a
discrete distribution in terms of Betti numbers or Euler characteristics,
without having to resort to smoothing or having to define a particular scale.
In this paper, we introduce the necessary mathematical background and
describe the method and implementation, while we address the application to 3D
simulated and observed data sets to the companion paper.Comment: A higher resolution version is available at
http://www.iap.fr/users/sousbie together with complementary material.
Submitted to MNRA
The fully connected N-dimensional skeleton: probing the evolution of the cosmic web
A method to compute the full hierarchy of the critical subsets of a density
field is presented. It is based on a watershed technique and uses a probability
propagation scheme to improve the quality of the segmentation by circumventing
the discreteness of the sampling. It can be applied within spaces of arbitrary
dimensions and geometry. This recursive segmentation of space yields, for a
-dimensional space, a succession of -dimensional subspaces that
fully characterize the topology of the density field. The final 1D manifold of
the hierarchy is the fully connected network of the primary critical lines of
the field : the skeleton. It corresponds to the subset of lines linking maxima
to saddle points, and provides a definition of the filaments that compose the
cosmic web as a precise physical object, which makes it possible to compute any
of its properties such as its length, curvature, connectivity etc... When the
skeleton extraction is applied to initial conditions of cosmological N-body
simulations and their present day non linear counterparts, it is shown that the
time evolution of the cosmic web, as traced by the skeleton, is well accounted
for by the Zel'dovich approximation. Comparing this skeleton to the initial
skeleton undergoing the Zel'dovich mapping shows that two effects are competing
during the formation of the cosmic web: a general dilation of the larger
filaments that is captured by a simple deformation of the skeleton of the
initial conditions on the one hand, and the shrinking, fusion and disappearance
of the more numerous smaller filaments on the other hand. Other applications of
the N dimensional skeleton and its peak patch hierarchy are discussed.Comment: Accepted for publication in MNRA
aski: full-sky lensing map-making algorithms
Within the context of upcoming full-sky lensing surveys, the edge-preserving non-linear algorithm aski (All-Sky κ Inversion) is presented. Using the framework of Maximum A Posteriori inversion, it aims at recovering the optimal full-sky convergence map from noisy surveys with masks. aski contributes two steps: (i) CCD images of possibly crowded galactic fields are deblurred using automated edge-preserving deconvolution; (ii) once the reduced shear is estimated using standard techniques, the partially masked convergence map is also inverted via an edge-preserving method. The efficiency of the deblurring of the image is quantified by the relative gain in the quality factor of the reduced shear, as estimated by SExtractor. Cross-validation as a function of the number of stars removed yields an automatic estimate of the optimal level of regularization for the deconvolution of the galaxies. It is found that when the observed field is crowded, this gain can be quite significant for realistic ground-based 8-m class surveys. The most significant improvement occurs when both positivity and edge-preserving ℓ1−ℓ2 penalties are imposed during the iterative deconvolution. The quality of the convergence inversion is investigated on noisy maps derived from the horizon-4πN-body simulation with a signal-to-noise ratio (S/N) within the range ℓcut= 500-2500, with and without Galactic cuts, and quantified using one-point statistics (S3 and S4), power spectra, cluster counts, peak patches and the skeleton. It is found that (i) the reconstruction is able to interpolate and extrapolate within the Galactic cuts/non-uniform noise; (ii) its sharpness-preserving penalization avoids strong biasing near the clusters of the map; (iii) it reconstructs well the shape of the PDF as traced by its skewness and kurtosis; (iv) the geometry and topology of the reconstructed map are close to the initial map as traced by the peak patch distribution and the skeleton's differential length; (v) the two-point statistics of the recovered map are consistent with the corresponding smoothed version of the initial map; (vi) the distribution of point sources is also consistent with the corresponding smoothing, with a significant improvement when ℓ1−ℓ2 prior is applied. The contamination of B modes when realistic Galactic cuts are present is also investigated. Leakage mainly occurs on large scales. The non-linearities implemented in the model are significant on small scales near the peaks in the fiel
Small-Angle CMB Temperature Anisotropies Induced by Cosmic Strings
We use Nambu-Goto numerical simulations to compute the cosmic microwave
background (CMB) temperature anisotropies induced at arcminute angular scales
by a network of cosmic strings in a Friedmann-Lemaitre-Robertson-Walker (FLRW)
expanding universe. We generate 84 statistically independent maps on a 7.2
degree field of view, which we use to derive basic statistical estimators such
as the one-point distribution and two-point correlation functions. At high
multipoles, the mean angular power spectrum of string-induced CMB temperature
anisotropies can be described by a power law slowly decaying as \ell^{-p}, with
p=0.889 (+0.001,-0.090) (including only systematic errors). Such a behavior
suggests that a nonvanishing string contribution to the overall CMB
anisotropies may become the dominant source of fluctuations at small angular
scales. We therefore discuss how well the temperature gradient magnitude
operator can trace strings in the context of a typical arcminute
diffraction-limited experiment. Including both the thermal and nonlinear
kinetic Sunyaev-Zel'dovich effects, the Ostriker-Vishniac effect, and the
currently favored adiabatic primary anisotropies, we find that, on such a map,
strings should be ``eye visible,'' with at least of order ten distinctive
string features observable on a 7.2 degree gradient map, for tensions U down to
GU \simeq 2 x 10^{-7} (in Planck units). This suggests that, with upcoming
experiments such as the Atacama Cosmology Telescope (ACT), optimal
non-Gaussian, string-devoted statistical estimators applied to small-angle CMB
temperature or gradient maps may put stringent constraints on a possible cosmic
string contribution to the CMB anisotropies.Comment: 17 pages, 9 figures. v2: matches published version, minor
clarifications added, typo in Eq. (8) fixed, results unchange
The M16 molecular complex under the influence of NGC6611. Herschel's perspective of the heating effect on the Eagle Nebula
We present Herschel images from the HOBYS key program of the Eagle Nebula
(M16) in the far-infrared and sub-millimetre, using the PACS and SPIRE cameras
at 70{\mu}m, 160{\mu}m, 250{\mu}m, 350{\mu}m, 500{\mu}m. M16, home to the
Pillars of Creation, is largely under the influence of the nearby NGC6611
high-mass star cluster. The Herschel images reveal a clear dust temperature
gradient running away from the centre of the cavity carved by the OB cluster.
We investigate the heating effect of NGC6611 on the entire M16 star-forming
complex seen by Herschel including the diffuse cloud environment and the dense
filamentary structures identified in this region. In addition, we interpret the
three-dimensional geometry of M16 with respect to the nebula, its surrounding
environment, and the NGC6611 cavity. The dust temperature and column density
maps reveal a prominent eastern filament running north-south and away from the
high-mass star-forming central region and the NGC6611 cluster, as well as a
northern filament which extends around and away from the cluster. The dust
temperature in each of these filaments decreases with increasing distance from
the NGC6611 cluster, indicating a heating penetration depth of \sim 10 pc in
each direction in 3 - 6 \times 10^{22} cm-2 column density filaments. We show
that in high-mass star-forming regions OB clusters impact the temperature of
future star-forming sites, modifying the initial conditions for collapse and
effecting the evolutionary criteria of protostars developed from spectral
energy distributions. Possible scenarios for the origin of the morphology seen
in this region are discussed, including a western equivalent to the eastern
filament, which was destroyed by the creation of the OB cluster and its
subsequent winds and radiation.Comment: 12 pages, including 3 appendix, 9 figures, accepted by A&
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