The nature of voids: I. Watershed void finders and their connection with theoretical models

TL;DR

This study analyzes voids in cosmological simulations using watershed algorithms, revealing discrepancies between observed void properties and theoretical models, and highlighting the influence of tracer sampling on void density measurements.

Contribution

It provides a detailed empirical analysis of void properties in simulations, connecting watershed void finders with theoretical models and exposing limitations of existing density profile fits.

Findings

Larger voids tend to have deeper density minima.

Tracer density at voids is usually lower than true density, depending on sampling.

Void density profiles do not match theoretical expectations.

Abstract

The statistical study of voids in the matter distribution promises to be an important tool for precision cosmology, but there are known discrepancies between theoretical models of voids and the voids actually found in large simulations or galaxy surveys. The empirical properties of observed voids are also not well understood. In this paper, we study voids in an N-body simulation, using the ZOBOV watershed algorithm. As in other studies, we use sets of subsampled dark matter particles as tracers to identify voids, but we use the full-resolution simulation output to measure dark matter densities at the identified locations. Voids span a wide range of sizes and densities, but there is a clear trend towards larger voids containing deeper density minima, a trend which is expected for all watershed void finders. We also find that the tracer density at void locations is usually smaller than the true density, and that this relationship depends on the sampling density of tracers. We show that fits given in the literature fail to match the observed density profiles of voids. The average enclosed density contrast within watershed voids varies widely with both the size of the void and the minimum density within it, but is always far from the shell crossing threshold expected from theoretical models. Voids with deeper density minima also show much broader density profiles. We discuss the implications of these results for the excursion set approach to modelling such voids.