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DESIVAST: A Catalog of Low-Redshift Voids using Data from the DESI DR1 Bright Galaxy Survey

This paper presents three void catalogs derived from the DESI Year 1 Bright Galaxy Survey using VoidFinder and V2 algorithms, identifying over 1,400 voids out to redshift z=0.24 and validating their consistency with SDSS data for future cosmological and environmental studies.

Original authors: Hernan Rincon, Segev BenZvi, Kelly Douglass, Dahlia Veyrat, Jessica Nicole Aguilar, Steven Ahlen, Davide Bianchi, David Brooks, Todd Claybaugh, Shaun Cole, Axel de la Macorra, Peter Doel, Andreu Font-

Published 2026-08-24

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CC BY 4.0

Original authors: Hernan Rincon, Segev BenZvi, Kelly Douglass, Dahlia Veyrat, Jessica Nicole Aguilar, Steven Ahlen, Davide Bianchi, David Brooks, Todd Claybaugh, Shaun Cole, Axel de la Macorra, Peter Doel, Andreu Font-Ribera, Jaime E. Forero-Romero, Enrique Gaztañaga, Satya Gontcho A Gontcho, Gaston Gutierrez, Klaus Honscheid, Cullan Howlett, Stephanie Juneau, Robert Kehoe, Sergey Koposov, Andrew Lambert, Martin Landriau, Laurent Le Guillou, Aaron Meisner, Ramon Miquel, John Moustakas, Gustavo Niz, Will Percival, Francisco Prada, Ignasi Pérez-Ràfols, Graziano Rossi, Eusebio Sanchez, Michael Schubnell, Hee-Jong Seo, David Sprayberry, Gregory Tarlé, Benjamin Alan Weaver, Hu Zou

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The universe is not a smooth, empty expanse. On the largest scales, matter arranges itself into a vast, intricate network known as the cosmic web. This structure consists of dense clusters of galaxies connected by long, thin filaments, all surrounding enormous, nearly empty bubbles called voids. These voids are not merely gaps; they are dynamic regions where gravity has pushed matter away, leaving behind vast underdensities that occupy most of the volume of the cosmos. Because these empty spaces are so large and their shapes are often roughly spherical, they serve as unique laboratories for testing our understanding of the universe's expansion and the mysterious force known as dark energy. By studying how these voids form and evolve, astronomers can probe the fundamental laws that govern the cosmos, offering a different perspective than looking at the dense clusters of galaxies alone.

A team of researchers has now taken a significant step forward in mapping these cosmic voids using data from the Dark Energy Spectroscopic Instrument, or DESI. This instrument is currently conducting a massive survey of the sky, measuring the distances to millions of galaxies to create a detailed three-dimensional map of the universe. The team, led by Hernan Rincon and colleagues, focused specifically on the "Bright Galaxy Survey" portion of the DESI data, which captures the closest and most luminous galaxies. They constructed three separate catalogs of voids in this nearby universe, reaching out to a distance where light has traveled for roughly 2.4 billion years. To ensure their maps were accurate, they used two different mathematical methods to find the voids: one that grows spheres into empty spaces until they hit galaxies, and another that treats the distribution of galaxies like a landscape of hills and valleys, identifying the low points as voids.

The result is a collection of nearly 1,500 voids found by the first method and several hundred more found by the second method. The researchers found that the two methods produce slightly different pictures of the same cosmic landscape. The first method identified smaller, more numerous voids, while the second method found fewer, but much larger, voids that often encompassed the smaller ones. This difference is not a mistake but a reflection of how each algorithm interprets the data; one is better at finding distinct, isolated empty regions, while the other is designed to fill the entire volume of the survey with voids, leaving almost no space unaccounted for. The team also compared their new DESI maps with older maps created from the Sloan Digital Sky Survey, a previous major project. They found that while the general locations of the voids matched up well, the exact boundaries and sizes of the empty spaces differed significantly.

These discrepancies arise largely from the different ways the two surveys observed the sky. The older survey covered a large, continuous patch of the northern sky, while the new DESI data, in its first release, covers a long, narrow strip. This difference in shape means that many of the voids found in the new data sit right on the edge of the survey area, making them harder to measure precisely. The researchers accounted for this by carefully defining which voids were safe to study and which were too close to the edge. They also discovered that the two surveys counted galaxies slightly differently due to variations in how they measured the brightness of stars and galaxies. This small difference in brightness measurements changed which galaxies were included in the final list, which in turn shifted the calculated sizes and locations of the voids. Notably, while theoretical models expect void sizes to increase as the universe expands (meaning larger voids at lower redshifts), previous high-redshift catalogs often found the opposite—abundances of very large voids—likely due to challenges in identifying low-density regions. The new low-redshift DESI catalogs avoid this issue by focusing on a volume-limited sample where the identification of these underdensities is most reliable.

Despite these technical differences, the study confirms that the new DESI data is a powerful tool for cosmology. The team found that the voids in their new catalog are consistent with expectations for low-redshift environments, avoiding the counter-intuitive trends seen in some high-redshift studies. They also projected that as the DESI survey continues and covers more of the sky, the number of identified voids will grow to over 15,000 for the smallest method and around 7,000 for the larger ones. This future expansion will allow astronomers to compare the two surveys over a much larger, shared area, removing the edge effects that currently complicate the comparison. For now, these catalogs provide a solid foundation for studying how galaxies change as they move through different cosmic environments, from the dense walls of the cosmic web to the quiet emptiness of the voids.

Technical Summary: DESIVAST: Catalogs of Low-Redshift Voids using Data from the DESI Data Release 1 Bright Galaxy Survey

Problem and Motivation
Cosmic voids, the vast underdense regions of the Universe, serve as unique dynamical objects and cosmological probes for dark energy and modified gravity. While previous void catalogs have been constructed using the Sloan Digital Sky Survey (SDSS), the Dark Energy Spectroscopic Instrument (DESI) offers a new benchmark with over 40 million targets. However, constructing void catalogs from DESI data presents challenges regarding survey target selection, tracer density variations with redshift, and the impact of survey masks. Specifically, weighting algorithms to account for tracer density can lead to the detection of artificially large voids, while volume-limited samples are preferred for accurately identifying low-density regions and studying galaxy properties as a function of environment. This paper addresses the need for low-redshift, volume-limited void catalogs using DESI Data Release 1 (DR1) to facilitate studies of galaxy-environment variation and cosmological tests.

Methodology
The authors constructed void catalogs using a volume-limited sample of the DESI Bright Galaxy Survey (BGS) Bright galaxies within a redshift limit of $z \le 0.24$ . The selection criteria included galaxies with synthesized SDSS $r$ -band absolute magnitudes $M_r \le -20$ , corrected for K-corrections and evolutionary effects ( $E$ -corrections) to $z=0.1$ . A rigorous angular mask was applied to ensure regions with up to four overlapping survey passes were observed, resulting in a final sample of 479,486 galaxies covering 2,320 deg² in the North Galactic Cap and 386 deg² in the South Galactic Cap.

Three distinct void catalogs were generated using two primary algorithms implemented in the Void Analysis Software Toolkit (VAST):

VoidFinder: A sphere-growing algorithm that partitions galaxies into "wall" and "field" regions based on the third-nearest neighbor distance. It identifies voids as "maximal spheres" bordered by wall galaxies.
V2 (ZOBOV): A watershed algorithm that uses Voronoi tessellation to define low-density basins ("zones"). Two pruning methods were applied:
- V2 $_{REVOLVER}$ : Treats all individual zones as voids without combining them.
- V2 $_{VIDE}$ : Combines adjacent zones based on a maximum linking density threshold and applies a central-density cut, resulting in a hierarchy of overlapping voids.

To mitigate survey boundary effects, the authors defined a fiducial volume ( $V_{fid}$ ) by excluding regions within 30 $Mpc\ h^{-1}$ of the survey edges. Statistical uncertainties were estimated using 25 AMTL mock catalogs derived from the AbacusSummit N-body simulation, while systematic uncertainties related to magnitude measurements were assessed by varying magnitude cuts based on the scatter between DESI and SDSS photometry.

Key Contributions and Results
The paper presents the DESIVAST DR1 value-added catalog, containing three void catalogs:

VoidFinder: 1,489 interior voids (3,765 total including edge voids).
V2 $_{REVOLVER}$ : 389 interior voids (1,992 total).
V2 $_{VIDE}$ : 297 interior voids (1,478 total, of which 249 are top-level "parent" voids that do not overlap with one another).

Key findings include:

Void Properties: VoidFinder voids are smaller and more numerous, with a median effective radius of $15.7 \pm 0.1$ $Mpc\ h^{-1}$ . V2 voids are larger, with median effective radii of $19.8 \pm 0.2$ $Mpc\ h^{-1}$ (REVOLVER) and $19.3 \pm 0.3$ $Mpc\ h^{-1}$ (VIDE).
Volume Filling: V2 algorithms fill a significantly larger fraction of the survey volume than VoidFinder. V2 $_{REVOLVER}$ identifies 98.7% of the fiducial volume as voids, whereas VoidFinder identifies 62.8%.
Galaxy Membership: Consistent with the volume filling, V2 $_{REVOLVER}$ places 98.6% of galaxies within voids, while VoidFinder places only 22.8%.
Comparison with SDSS: The authors compared DESIVAST catalogs with reprocessed SDSS DR7 void catalogs. While the median void radii and volume fractions agreed within uncertainties, significant differences were observed in the volume overlap between the two surveys. For VoidFinder, 50.1% of the common volume was identified as void in both surveys, whereas for V2 $_{VIDE}$ , only 35.8% overlapped.
Edge Effects: Due to the thin angular mask of DESI DR1, over 60% of detected voids in the initial catalogs were classified as "edge voids." The fiducial volume cut was essential for statistical analysis.

Significance and Claims
The authors state that these catalogs are suitable for studying the variation of galaxy properties with cosmic environment and for cosmological studies. They emphasize that while the void properties (size, volume fraction) are generally consistent with SDSS results, the differences in volume overlap are attributed to variations in galaxy selection, survey masks, and completeness between the two surveys.

The paper modestly claims that despite the disagreement in volume overlap, VoidFinder demonstrates resilience in identifying the same general low-density regions across different survey designs. The authors note that the current DR1 catalogs are limited by the survey mask but project that a complete DESI survey will yield approximately 15,000 interior VoidFinder voids and 7,000 interior V2 voids, allowing for more robust comparisons of interior voids between DESI and SDSS. The catalogs are intended to be updated with each subsequent DESI data release.

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