The Spin and Orientation of Dark Matter Halos Within Cosmic Filaments

Abstract

Clusters, filaments, sheets and voids are the building blocks of the cosmic web. Forming dark matter halos respond to these different large-scale environments, and this in turn affects the properties of galaxies hosted by the halos. It is therefore important to understand the systematic correlations of halo properties with the morphology of the cosmic web, as this informs both about galaxy formation physics and possible systematics of weak lensing studies. In this study, we present and compare two distinct algorithms for finding cosmic filaments and sheets, a task which is far less well established than the identification of dark matter halos or voids. One method is based on the smoothed dark matter density field, the other uses the halo distributions directly. We apply both techniques to one high resolution N-body simulation and reconstruct the filamentary/sheet like network of the dark matter density field. We focus on investigating the properties of the dark matter halos inside these structures, in particular on the directions of their spins and the orientation of their shapes with respect to the directions of the filaments and sheets. We find that both the spin and the major axes of filament-halos with masses 10 13 h −1 M ⊙ are preferentially aligned with the direction of the filaments. The spins and major axes of halos in sheets tend to lie parallel to the sheets. There is an opposite mass dependence of the alignment strengths for the spin (negative) and major (positive) axes, i.e. with increasing halo mass the major axis tends to be more strongly aligned with the direction of the filament whereas the alignment between halo spin and filament becomes weaker with increasing halo mass. The alignment strengths as a function of distance to the most massive node halo indicate that there is a transit large scale environment impact: from the 2-D collapse phase of the filament to the 3-D collapse phase of the cluster/node halo at small separation. Overall, the two algorithms for filament/sheet identification investigated here agree well with each other. The method based on halos alone can be easily adapted for use with observational data sets. Subject headings: methods: data analysis -dark matter -large-scale structure of universe -galaxies: halos

The Local Group in the Cosmic Web

“…The absence of alignment for the spins is consistent with different studies of spin alingnment that mark the range 10 12 M as either a transition mass from alignment into anti-alignment or from no-alignment into alignment (Hahn et al 2007;Aragón-Calvo et al 2007;Zhang et al 2009;Codis et al 2012;Trowland et al 2013;Libeskind et al 2013;Forero-Romero et al 2014). We refer the reader to Table 2 in Forero-Romero et al (2014) for a review on the different results of spin alignment with the cosmic web in cosmological simulations.…”

The strongest gravitational lenses

“…Consequently, their lens model produces surface mass densities with different properties, which are decisive for the strength of the boost of the strong-lensing efficiency during cluster mergers. -Analyses of cluster mergers in numerical simulations reveal that (1) the major axes of infalling substructures are intrinsically aligned with the major axis of the main halo due to its tidal field and (2) subhaloes preferentially approach the main halo along its major axis (Lee et al 2005;Altay et al 2006;Zhang et al 2009). Motivated by these findings, Fedeli et al (2006) throughout modelled mergers by perfectly aligning the major axes of both haloes and assuming that the subhalo always approaches the main halo exactly along its major axis.…”

Evolution of Mass and Velocity Field in the Cosmic Web: Comparison Between Baryonic and Dark Matter

“…Various numerical methods have been developed to identify morphological components of the cosmic web in both simulation samples and observed galaxy distribution (Aragon-Calvo et al 2007a;Hahn et al 2007;Forero-Romero et al2009;Bond et al 2010a;Sousbie 2011;Hoffman et al 2012;Cautun et al 2013). Of particular interest is to reconstruct the large scale filamentary structures in observations and simulations, and study statistical properties of their lengths and widths, and alignment with galaxies and halos (Colberg et al 2005;Zhang et al 2009;Sousbie et al 2011;Tempel et al 2014;Gheller et al 2015). With the aid of numerical simulations, the mass and volume content, the density distribution, and the spatial extent of cosmic web components are investigated in detail, as well as their redshift evolution(for details see e.g.…”

“…Aragon-Calvo et al 2007b;Codis et al 2012;Libeskind et al 2013;Wang et al 2013;Dubois et al 2014), in order to interpreting the alignment of galaxies shape and distribution in different cosmic environment revealed by observation and simulations samples(e.g. Lee & Erdogdu 2007;Zhang et al 2009;Tempel & Libeskind 2013;Dong et al 2014;Zhang et al 2015). Nevertheless, investigation on the evolution of flows of baryonic and cold dark matter during the formation of cosmic web in simulations is inadequate, in comparison with the mass content.…”