Tensor anisotropy as a tracer of cosmic voids

TL;DR

This paper introduces a novel method using fractional anisotropy derived from eigenvalues of cosmic web tensors to identify and analyze cosmic voids in simulations, revealing their properties and potential universality.

Contribution

The study presents a new void detection technique based on fractional anisotropy without shape assumptions, and demonstrates its effectiveness on cosmological simulation data.

Findings

Void sizes differ between velocity shear and tidal webs.

Approximately 65% of voids are subcompensated, 35% overcompensated.

Universal density, velocity, and FA profiles are suggested when normalized by effective radius.

Abstract

We present a new method to find voids in cosmological simulations based on the tidal and the velocity shear tensors definitions of the cosmic web. We use the fractional anisotropy (FA) computed from the eigenvalues of each web scheme as a void tracer. We identify voids using a watershed transform based on the local minima of the FA field without making any assumption on the shape or structure of the voids. We test the method on the Bolshoi simulation and report on the abundance and radial averaged profiles for the density, velocity and fractional anisotropy. We find that voids in the velocity shear web are smaller than voids in the tidal web, with a particular overabundance of very small voids in the inner region of filaments/sheets. We classify voids as subcompensated/overcompansated depending on the absence/presence of an overdense matter ridge in their density profile, finding that close to $65\%$ and $35\%$ of the total population are classified into each category, respectively. Finally, we find evidence for the existence of universal profiles from the radially averaged profiles for density, velocity and fractional anisotropy. This requires that the radial coordinate is normalized to the effective radius of each void. Put together, all these results show that the FA is a reliable tracer for voids, which can be used in complementarity to other existing methods and tracers.