ABSTRACT
We investigate the alignment of the spin of dark matter haloes relative (i) to the surrounding large‐scale filamentary structure, and (ii) to the tidal tensor eigenvectors using the Horizon ...4π dark matter simulation which resolves over 43 million dark matter haloes at redshift zero. We detect a clear mass transition: the spin of dark matter haloes above a critical mass
tends to be perpendicular to the closest large‐scale filament (with an excess probability of up to 12 per cent), and aligned with the intermediate axis of the tidal tensor (with an excess probability of up to 40 per cent), whereas the spin of low‐mass haloes is more likely to be aligned with the closest filament (with an excess probability of up to 15 per cent). Furthermore, this critical mass is redshift‐dependent, scaling as M crit s(z)≈M0s(1+z)−γs with γs = 2.5 ± 0.2. A similar fit for the redshift evolution of the tidal tensor transition mass yields
and γt = 3 ± 0.3. This critical mass also varies weakly with the scale defining filaments.
We propose an interpretation of this signal in terms of large‐scale cosmic flows. In this picture, most low‐mass haloes 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 haloes 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 measured excess probabilities of alignment with respect to the eigendirections 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. as it extends it by characterizing the geometry of secondary infall at high redshift.
Tracing the cosmic web Libeskind, Noam I.; van de Weygaert, Rien; Cautun, Marius ...
Monthly Notices of the Royal Astronomical Society,
01/2018, Letnik:
473, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The cosmic web is one of the most striking features of the distribution of galaxies and dark matter on the largest scales in the Universe. It is composed of dense regions packed full of galaxies, ...long filamentary bridges, flattened sheets and vast low-density voids. The study of the cosmic web has focused primarily on the identification of such features, and on understanding the environmental effects on galaxy formation and halo assembly. As such, a variety of different methods have been devised to classify the cosmic web – depending on the data at hand, be it numerical simulations, large sky surveys or other. In this paper, we bring 12 of these methods together and apply them to the same data set in order to understand how they compare. In general, these cosmic-web classifiers have been designed with different cosmological goals in mind, and to study different questions. Therefore, one would not a priori expect agreement between different techniques; however, many of these methods do converge on the identification of specific features. In this paper, we study the agreements and disparities of the different methods. For example, each method finds that knots inhabit higher density regions than filaments, etc. and that voids have the lowest densities. For a given web environment, we find a substantial overlap in the density range assigned by each web classification scheme. We also compare classifications on a halo-by-halo basis; for example, we find that 9 of 12 methods classify around a third of group-mass haloes (i.e. M_halo ∼ 10^13.5 h^−1 M_⊙) as being in filaments. Lastly, so that any future cosmic-web classification scheme can be compared to the 12 methods used here, we have made all the data used in this paper public.
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 Poincaré 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 65–67 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.
We present the first Herschel PACS and SPIRE results of the Vela C molecular complex in the far-infrared and submillimetre regimes at 70, 160, 250, 350, and 500 μm, spanning the peak of emission of ...cold prestellar or protostellar cores. Column density and multi-resolution analysis (MRA) differentiates the Vela C complex into five distinct sub-regions. Each sub-region displays differences in their column density and temperature probability distribution functions (PDFs), in particular, the PDFs of the “Centre-Ridge” and “South-Nest” sub-regions appear in stark contrast to each other. The Centre-Ridge displays a bimodal temperature PDF representative of hot gas surrounding the HII region RCW 36 and the cold neighbouring filaments, whilst the South-Nest is dominated by cold filamentary structure. The column density PDF of the Centre-Ridge is flatter than the South-Nest, with a high column density tail, consistent with formation through large-scale flows, and regulation by self-gravity. At small to intermediate scales MRA indicates the Centre-Ridge to be twice as concentrated as the South-Nest, whilst on larger scales, a greater portion of the gas in the South-Nest is dominated by turbulence than in the Centre-Ridge. In Vela C, high-mass stars appear to be preferentially forming in ridges, i.e., dominant high column density filaments.
We report on a new merging group of galaxies, Suzaku J1552+2739, at z ~ 0.08 revealed by a Suzaku observation. The group was found by observing a junction of galaxy filaments optically identified in ...the Sloan Digital Sky Survey spectroscopic data. Suzaku J1552+2739 exhibits an irregular morphology and presents several peaks in its X-ray image. A bright elliptical galaxy, observable in the central peak, allows the localization of the group at z = 0.083. We found a significant hot spot visible in the X-ray hardness map, close to the second peak. The spectroscopic temperature is T = 1.6+0.4 --0.1 keV within R 500 = 0.6 Mpc and T = 3-5 keV in the hot spot. We interpret those results as Suzaku J1552+2739 being located in the center of a major merging process. The observation of a galaxy group showing multiple X-ray peaks and a hot spot at the same time is rare and we believe in particular that the study of Suzaku J1552+2739 is potentially of significant interest for better understanding the dynamical and thermal evolution of the intragroup and intracluster medium, as well as its relation with the surrounding environment.
Recent algorithms with state-of-the-art few-shot classification results start their procedure by computing data features output by a large pretrained model. In this paper we systematically ...investigate which models provide the best representations for a few-shot image classification task when pretrained on the Imagenet dataset. We test their representations when used as the starting point for different few-shot classification algorithms. We observe that models trained on a supervised classification task have higher performance than models trained in an unsupervised manner even when transferred to out-of-distribution datasets. Models trained with adversarial robustness transfer better, while having slightly lower accuracy than supervised models.
We present the DIScrete PERsistent Structures Extractor (DisPerSE), an open source software for the automatic and robust identification of structures in 2D and 3D noisy data sets. The software is ...designed to identify all sorts of topological structures, such as voids, peaks, sources, walls and filaments through segmentation, with a special emphasis put on the later ones. Based on discrete Morse theory, DisPerSE is able to deal directly with noisy datasets using the concept of persistence (a measure of the robustness of topological features) and can be applied indifferently to various sorts of data-sets defined over a possibly bounded manifold : 2D and 3D images, structured and unstructured grids, discrete point samples via the delaunay tesselation, Healpix tesselations of the sphere, ... Although it was initially developed with cosmology in mind, various I/O formats have been implemented and the current version is quite versatile. It should therefore be useful for any application where a robust structure identification is required as well as for studying the topology of sampled functions (e.g. computing persistent Betti numbers). DisPerSE can be downloaded directly from the website http://www2.iap.fr/users/sousbie/ and a thorough online documentation is also available at the same address.
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 Poincaré 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.
PDF
