Structure in the 3D Galaxy Distribution: II. Voids and Watersheds of Local Maxima and Minima

TL;DR

This study compares 12 different geometrical methods for defining cosmic structures like voids and watersheds in galaxy distributions, revealing continuous multi-scale size distributions without discrete hierarchy.

Contribution

It introduces a parameter-free HOP algorithm for structure assignment and a novel Delaunay tetrahedra-based void finder, enhancing the analysis of cosmic structures.

Findings

Sizes of structures follow continuous multi-scale distributions

No evidence of discrete hierarchical organization in galaxy structures

New void detection method based on Delaunay tetrahedra

Abstract

The major uncertainties in studies of the multi-scale structure of the Universe arise not from observational errors but from the variety of legitimate definitions and detection methods for individual structures. To facilitate the study of these methodological dependencies we have carried out 12 different analyses defining structures in various ways. This has been done in a purely geometrical way by utilizing the HOP algorithm as a unique parameter-free method of assigning groups of galaxies to local density maxima or minima. From three density estimation techniques (smoothing kernels, Bayesian Blocks and self organizing maps) applied to three data sets (the Sloan Digital Sky Survey Data Release 7, the Millennium Simulation and randomly distributed points) we tabulate information that can be used to construct catalogs of structures connected to local density maxima and minima. The resulting sizes follow continuous multi-scale distributions with no indication of the presence of a discrete hierarchy. We also introduce a novel void finder that utilizes a method to assemble Delaunay tetrahedra into connected structures and characterizes regions very nearly empty of galaxies in the source catalog.