A dynamical classification of the cosmic web

Abstract

In this paper, we propose a new dynamical classification of the cosmic web. Each point in space is classified in one of four possible web types: voids, sheets, filaments and knots. The classification is based on the evaluation of the deformation tensor (i.e. the Hessian of the gravitational potential) on a grid. The classification is based on counting the number of eigenvalues above a certain threshold, λth, at each grid point, where the case of zero, one, two or three such eigenvalues corresponds to void, sheet, filament or a knot grid point. The collection of neighbouring grid points, friends of friends, of the same web type constitutes voids, sheets, filaments and knots as extended web objects. A simple dynamical consideration of the emergence of the web suggests that the threshold should not be null, as in previous implementations of the algorithm. A detailed dynamical analysis would have found different threshold values for the collapse of sheets, filaments and knots. Short of such an analysis a phenomenological approach has been opted for, looking for a single threshold to be determined by analysing numerical simulations. Our cosmic web classification has been applied and tested against a suite of large (dark matter only) cosmological N‐body simulations. In particular, the dependence of the volume and mass filling fractions on λth and on the resolution has been calculated for the four web types. We also study the percolation properties of voids and filaments. Our main findings are as follows. (i) Already at λth= 0.1 the resulting web classification reproduces the visual impression of the cosmic web. (ii) Between 0.2 ≲λth≲ 0.4, a system of percolated voids coexists with a net of interconnected filaments. This suggests a reasonable choice for λth as the parameter that defines the cosmic web. (iii) The dynamical nature of the suggested classification provides a robust framework for incorporating environmental information into galaxy formation models, and in particular to semi‐analytical models.

The alignment of galaxy spin with the shear field in observations

“…For the purpose of checking galaxy alignments, a measure of the tidal shear field (e.g. Hahn et al 2007;Forero-Romero et al 2009;Hoffman et al 2012) is needed.…”

“…Methods to quantify the cosmic web abound (Stoica et al 2005;Shen et al 2006;Aragón-Calvo et al 2007;Hahn et al 2007;Sousbie et al 2008;Forero-Romero et al 2009;Hoffman et al 2012;Libeskind et al 2012Libeskind et al , 2013Tempel et al 2014a) depending on the data and problem at hand. In general, the methods based on simulations rely on a more complete knowledge of the cosmic fluid than what is usually available in observations.…”

The cosmic web for density perturbations of various scales

“…It is natural to expect that the web of rich superclusters is formed by density perturbations of scale 100 h −1 Mpc and above. The skeleton of the cosmic web is discussed by, among others, Sheth & van de Weygaert (2004), Hahn et al (2007), Forero-Romero et al (2009), Sousbie et al (2008Sousbie et al ( , 2009, Aragón-Calvo et al (2010b), Bond et al (2010b,a), and Einasto et al (2011). Thus, we can say that the role of smallscale and large-scale density perturbations is relatively well understood.…”

Angular momentum evolution in dark matter haloes: a study of the Bolshoi and Millennium simulations

“…In this section we study this issue using the Bolshoi simulation, classifying the environment of haloes into voids, walls, filaments and knots, using the T-Web algorithm (Forero-Romero et al 2009;Hoffman et al 2012) publicly available in the CosmoSim virtual observatory 3 . The T-Web is defined as the Hessian of the gravitational potential, and is computed on a 256 3 cubic grid, which in the case of Bolshoi corresponds to cells with ∼ 1h −1 M pc a side.…”