13 research outputs found
ColDICE: a parallel Vlasov-Poisson solver using moving adaptive simplicial tessellation
Resolving numerically Vlasov-Poisson equations for initially cold systems can
be reduced to following the evolution of a three-dimensional sheet evolving in
six-dimensional phase-space. We describe a public parallel numerical algorithm
consisting in representing the phase-space sheet with a conforming,
self-adaptive simplicial tessellation of which the vertices follow the
Lagrangian equations of motion. The algorithm is implemented both in six- and
four-dimensional phase-space. Refinement of the tessellation mesh is performed
using the bisection method and a local representation of the phase-space sheet
at second order relying on additional tracers created when needed at runtime.
In order to preserve in the best way the Hamiltonian nature of the system,
refinement is anisotropic and constrained by measurements of local Poincar\'e
invariants. Resolution of Poisson equation is performed using the fast Fourier
method on a regular rectangular grid, similarly to particle in cells codes. To
compute the density projected onto this grid, the intersection of the
tessellation and the grid is calculated using the method of Franklin and
Kankanhalli (1993) generalised to linear order. As preliminary tests of the
code, we study in four dimensional phase-space the evolution of an initially
small patch in a chaotic potential and the cosmological collapse of a
fluctuation composed of two sinusoidal waves. We also perform a "warm" dark
matter simulation in six-dimensional phase-space that we use to check the
parallel scaling of the code.Comment: Code and illustration movies available at:
http://www.vlasix.org/index.php?n=Main.ColDICE - Article submitted to Journal
of Computational Physic
The persistent cosmic web and its filamentary structure II: Illustrations
The recently introduced discrete persistent structure extractor (DisPerSE,
Soubie 2010, paper I) is implemented on realistic 3D cosmological simulations
and observed redshift catalogues (SDSS); it is found that DisPerSE traces
equally well the observed filaments, walls, and voids in both cases. In either
setting, filaments are shown to connect onto halos, outskirt walls, which
circumvent voids. Indeed this algorithm operates directly on the particles
without assuming anything about the distribution, and yields a natural
(topologically motivated) self-consistent criterion for selecting the
significance level of the identified structures. It is shown that this
extraction is possible even for very sparsely sampled point processes, as a
function of the persistence ratio. Hence astrophysicists should be in a
position to trace and measure precisely the filaments, walls and voids from
such samples and assess the confidence of the post-processed sets as a function
of this threshold, which can be expressed relative to the expected amplitude of
shot noise. In a cosmic framework, this criterion is comparable to friend of
friend for the identifications of peaks, while it also identifies the connected
filaments and walls, and quantitatively recovers the full set of topological
invariants (Betti numbers) {\sl directly from the particles} as a function of
the persistence threshold. This criterion is found to be sufficient even if one
particle out of two is noise, when the persistence ratio is set to 3-sigma or
more. The algorithm is also implemented on the SDSS catalogue and used to locat
interesting configurations of the filamentary structure. In this context we
carried the identification of an ``optically faint'' cluster at the
intersection of filaments through the recent observation of its X-ray
counterpart by SUZAKU. The corresponding filament catalogue will be made
available online.Comment: A higher resolution version is available at
http://www.iap.fr/users/sousbie together with complementary material (movie
and data). Submitted to MNRA
The Skeleton: Connecting Large Scale Structures to Galaxy Formation
We report on two quantitative, morphological estimators of the filamentary
structure of the Cosmic Web, the so-called global and local skeletons. The
first, based on a global study of the matter density gradient flow, allows us
to study the connectivity between a density peak and its surroundings, with
direct relevance to the anisotropic accretion via cold flows on galactic halos.
From the second, based on a local constraint equation involving the
derivatives of the field, we can derive predictions for powerful statistics,
such as the differential length and the relative saddle to extrema counts of
the Cosmic web as a function of density threshold (with application to
percolation of structures and connectivity), as well as a theoretical framework
to study their cosmic evolution through the onset of gravity-induced
non-linearities.Comment: 10 pages, 8 figures; proceedings of the "Invisible Universe" 200
Connecting the cosmic web to the spin of dark halos: implications for galaxy formation
We investigate the alignment of the spin of dark matter halos relative (i) to
the surrounding large-scale filamentary structure, and (ii) to the tidal tensor
eigenvectors using the Horizon 4pi dark matter simulation which resolves over
43 million dark matter halos at redshift zero.
We detect a clear mass transition: the spin of dark matter halos above a
critical mass tends to be perpendicular to the closest filament, and aligned
with the intermediate axis of the tidal tensor, whereas the spin of low-mass
halos is more likely to be aligned with the closest filament. Furthermore, this
critical mass of 5 10^12 is redshift-dependent and scales as (1+z)^-2.5. We
propose an interpretation of this signal in terms of large-scale cosmic flows.
In this picture, most low-mass halos are formed through the winding of flows
embedded in misaligned walls; hence they acquire a spin parallel to the axis of
the resulting filaments forming at the intersection of these walls. On the
other hand, more massive halos are typically the products of later mergers
along such filaments, and thus they acquire a spin perpendicular to this
direction when their orbital angular momentum is converted into spin. We show
that this scenario is consistent with both the measured excess probabilities of
alignment w.r.t. the eigen-directions of the tidal tensor, and halo merger
histories. On a more qualitative level, it also seems compatible with 3D
visualization of the structure of the cosmic web as traced by "smoothed" dark
matter simulations or gas tracer particles. Finally, it provides extra support
to the disc forming paradigm presented by Pichon et al (2011) as it extends it
by characterizing the geometry of secondary infall at high redshift.Comment: 18 pages; 14 figures; accepted by MNRA
Swirling around filaments: are large-scale structure vortices spinning up dark halos?
The kinematic analysis of dark matter and hydrodynamical simulations suggests
that the vorticity in large-scale structure is mostly confined to, and
predominantly aligned with their filaments, with an excess of probability of 20
per cent to have the angle between vorticity and filaments direction lower than
60 degrees relative to random orientations. The cross sections of these
filaments are typically partitioned into four quadrants with opposite vorticity
sign, arising from multiple flows, originating from neighbouring walls. The
spins of halos embedded within these filaments are consistently aligned with
this vorticity for any halo mass, with a stronger alignment for the most
massive structures up to an excess of probability of 165 per cent. On large
scales, adiabatic/cooling hydrodynamical simulations display the same vorticity
in the gas as in the dark matter. The global geometry of the flow within the
cosmic web is therefore qualitatively consistent with a spin acquisition for
smaller halos induced by this large-scale coherence, as argued in Codis et al.
(2012). In effect, secondary anisotropic infall (originating from the
vortex-rich filament within which these lower-mass halos form) dominates the
angular momentum budget of these halos. The transition mass from alignment to
orthogonality is related to the size of a given multi-flow region with a given
polarity. This transition may be reconciled with the standard tidal torque
theory if the latter is augmented so as to account for the larger scale
anisotropic environment of walls and filaments.Comment: 17 pages, 19 figures, 3 tables. accepted for publication in MNRA
LE SQUELETTE DE L'UNIVERS: Un outil d'analyse topologique des grandes structures
La distribution de la matière dans l'Univers est supposée homogène et isotrope à très grande échelle mais l'observation de la distribution des galaxies lorsde grandes campagnes de recensements comme le SDSS nous montre un véritable réseau d'amas et de filaments sur des échelles de plusieurs centaines demégaparsecs.\\De nombreuses méthodes ont été développées dans le but de caractériser cette distribution et nous nous proposons dans cette thèse de présenter l'adaptation en trois dimensions d'un nouvel outil: le squelette.Cette méthode vise à donner une définition mathématique claire des filaments ainsi qu'un algorithme numérique robuste permettant leur identification ainsi que le calcul de leurs propriétés. \\Afin de pouvoir comparer les résultats obtenus à partir des simulations N-corps de matière noire aux observations, une nouvelle méthode, baptisée MoLUSC, spécialisée dans la création de catalogues virtuels degalaxies a aussi été élaborée. Elle se base sur les modèles semi-analytiques et est particulièrement efficace pour la fabrication de catalogues de grandetaille simulant de manière suffisamment réaliste les propriétés galactiques.\\Les utilisations de ces deux outils sont nombreuses et nous montrons par exemple qu'il est possible en mesurant la densité de longueur des filaments àune échelle donnée de contraindre la quantité de matière dans l'univers. Ces méthodes peuvent aussi être appliquées avec succès à la mesure statistique des propriétés du flux de matière noire le long des filaments, une mesure inédite. Nous présentons enfin de nombreuses applications possibles dont les résultats préliminaires sont très encourageants
The Skeleton: Connecting Large Scale Structures to Galaxy Formation
International audienceWe report on two quantitative, morphological estimators of the filamentary structure of the Cosmic Web, the so-called global and local skeletons. The first, based on a global study of the matter density gradient flow, allows us to study the connectivity between a density peak and its surroundings, with direct relevance to the anisotropic accretion via cold flows on galactic halos. From the second, based on a local constraint equation involving the derivatives of the field, we can derive predictions for powerful statistics, such as the differential length and the relative saddle to extrema counts of the Cosmic web as a function of density threshold (with application to percolation of structures and connectivity), as well as a theoretical framework to study their cosmic evolution through the onset of gravity-induced non-linearities