Genus statistics using the Delaunay tessellation field estimation method: (I) tests with the Millennium Simulation and the SDSS DR7

We study the topology of cosmic large-scale structure through the genus statistics, using galaxy catalogues generated from the Millennium Simulation and observational data from the latest Sloan Digital Sky Survey Data Release (SDSS DR7). We introduce a new method for constructing galaxy density fields and for measuring the genus statistics of its isodensity surfaces. It is based on a Delaunay tessellation field estimation (DTFE) technique that allows the definition of a piece-wise continuous density field and the exact computation of the topology of its polygonal isodensity contours, without introducing any free numerical parameter. Besides this new approach, we also employ the traditional approaches of smoothing the galaxy distribution with a Gaussian of fixed width, or by adaptively smoothing with a kernel that encloses a constant number of neighboring galaxies. Our results show that the Delaunay-based method extracts the largest amount of topological information. Unlike the traditional approach for genus statistics, it is able to discriminate between the different theoretical galaxy catalogues analyzed here, both in real space and in redshift space, even though they are based on the same underlying simulation model. In particular, the DTFE approach detects with high confidence a discrepancy of one of the semi-analytic models studied here compared with the SDSS data, while the other models are found to be consistent.Comment: 14 pages, 9 figures, accepted by Ap

The Spine of the Cosmic Web

We present the SpineWeb framework for the topological analysis of the Cosmic Web and the identification of its walls, filaments and cluster nodes. Based on the watershed segmentation of the cosmic density field, the SpineWeb method invokes the local adjacency properties of the boundaries between the watershed basins to trace the critical points in the density field and the separatrices defined by them. The separatrices are classified into walls and the spine, the network of filaments and nodes in the matter distribution. Testing the method with a heuristic Voronoi model yields outstanding results. Following the discussion of the test results, we apply the SpineWeb method to a set of cosmological N-body simulations. The latter illustrates the potential for studying the structure and dynamics of the Cosmic Web.Comment: Accepted for publication HIGH-RES version: http://skysrv.pha.jhu.edu/~miguel/SpineWeb

Fossil evidence for spin alignment of SDSS galaxies in filaments

We search for and find fossil evidence that the distribution of the spin axes of galaxies in cosmic web filaments relative to their host filaments are not randomly distributed. This would indicate that the action of large scale tidal torques effected the alignments of galaxies located in cosmic filaments. To this end, we constructed a catalogue of clean filaments containing edge-on galaxies. We started by applying the Multiscale Morphology Filter (MMF) technique to the galaxies in a redshift-distortion corrected version of the Sloan Digital Sky Survey DR5. From that sample we extracted those 426 filaments that contained edge-on galaxies (b/a < 0.2). These filaments were then visually classified relative to a variety of quality criteria. Statistical analysis using "feature measures" indicates that the distribution of orientations of these edge-on galaxies relative to their parent filament deviate significantly from what would be expected on the basis of a random distribution of orientations. The interpretation of this result may not be immediately apparent, but it is easy to identify a population of 14 objects whose spin axes are aligned perpendicular to the spine of the parent filament (\cos \theta < 0.2). The candidate objects are found in relatively less dense filaments. This might be expected since galaxies in such locations suffer less interaction with surrounding galaxies, and consequently better preserve their tidally induced orientations relative to the parent filament. The technique of searching for fossil evidence of alignment yields relatively few candidate objects, but it does not suffer from the dilution effects inherent in correlation analysis of large samples.Comment: 20 pages, 19 figures, slightly revised and upgraded version, accepted for publication by MNRAS. For high-res version see http://www.astro.rug.nl/~weygaert/SpinAlignJones.rev.pd

Numerical studies of the interstellar medium on galactic scales

Interstellar matter and star formatio

Spinning galaxies within the large scale structure of the Universe

Finding links between the large scale structure of the Universe and galaxy formation presents an important challenge for cosmology. The properties of dark matter halos in N-body simulations, in particular the spin angular momentum, can provide these links. This thesis is an in depth study of the alignments of halo spin direction within filaments in the large scale structure. Filaments in the halo and galaxy distribution of the Millennium simulation were identified using two simple methods and a difference between the spin orientation of low and high mass halos with the axis of filaments was uncovered. The evolution of these alignments and other aspects of halo spin suggested an ongoing process of angular momentum acquisition. This process was found to be largely reliant on the anisotropic infall of satellite halos. The spin of dark matter halos tends to become increasingly parallel to the axis of filaments and this change is driven by major mergers between halos traveling orthogonal to the axis of filaments. This new scenario of the build-up of dark matter halo spin could see significant consequences in theories of galaxy formation

Particle hydrodynamics with tessellation techniques

Lagrangian smoothed particle hydrodynamics (SPH) is a well-established approach to model fluids in astrophysical problems, thanks to its geometric flexibility and ability to automatically adjust the spatial resolution to the clumping of matter. However, a number of recent studies have emphasized inaccuracies of SPH in the treatment of fluid instabilities. The origin of these numerical problems can be traced back to spurious surface effects across contact discontinuities, and to SPH's inherent prevention of mixing at the particle level. We here investigate a new fluid particle model where the density estimate is carried out with the help of an auxiliary mesh constructed as the Voronoi tessellation of the simulation particles instead of an adaptive smoothing kernel. This Voronoi-based approach improves the ability of the scheme to represent sharp contact discontinuities. We show that this eliminates spurious surface tension effects present in SPH and that play a role in suppressing certain fluid instabilities. We find that the new `Voronoi Particle Hydrodynamics' described here produces comparable results than SPH in shocks, and better ones in turbulent regimes of pure hydrodynamical simulations. We also discuss formulations of the artificial viscosity needed in this scheme and how judiciously chosen correction forces can be derived in order to maintain a high degree of particle order and hence a regular Voronoi mesh. This is especially helpful in simulating self-gravitating fluids with existing gravity solvers used for N-body simulations.Comment: 26 pages, 24 figures, currentversion is accepted by MNRA

Simulations and observables in relativistic cosmology

The huge leap in volume and precision that will be achieved by upcoming large sky surveys will make our observables sensitive to a number of effects previously ignored, such as relativistic effects. These can potentially represent new systematics to take into account, but also new probes for our cosmological models. In this thesis, we explore novel tools to model these type of effects, with a particular emphasis on frame-dragging --- the leading order post-Newtonian effect --- in cosmological $N$-body simulations. In the first part, we discuss the implementation of a new code for general-relativistic simulations in cosmology, {\sc gramses}. The code is built upon the numerical infrastructure of the {\sc ramses} code, and implements a constrained formulation of general relativity in which scalar and vector modes of the spacetime metric are calculated fully nonlinearly. We perform several tests against both theory and well-established, state-of-the-art relativistic codes, demonstrating that {\sc gramses} is able to produce robust results. Furthermore, we introduce a new, general method to generate initial conditions for particles, which circumvents the gauge issues affecting the standard prescriptions. In the second part, we focus on exploring the impact of frame-dragging on different scenarios. Firstly, based on a high-resolution run with {\sc gramses}, we explore the behaviour of this effect in different types of dark matter haloes. In particular, we show that, although the gravitomagnetic force acting on dark matter is small relative to the Newtonian force, it can be up to one order of magnitude larger than previous literature results. Finally, we explore the possibility of detecting the gravitomagnetic effect that appears in lensing convergence maps via cross-correlations with the kinetic Sunyaev-Zel'dovich effect, which is imprinted in {\sc cmb} maps. We make forecasts for next-generation weak-lensing surveys such as {\sc euclid} and {\sc lsst}, and {\sc cmb} experiments such as Simons Observatory and {\sc cmb-s4}, and find that the gravitomagnetic effect can be detected on small angular scales, provided that several foreground contaminations can be reliably removed