🔭 astrophysics

The DESI DR1 Peculiar Velocity Survey: Fundamental Plane Catalogue

This paper presents the largest single set of low-redshift peculiar velocity measurements to date, derived from a Fundamental Plane catalogue of 98,292 early-type galaxies in the DESI DR1, which doubles the total number of such distances available and achieves a 26% precision in distance measurements.

Original authors: C. E. Ross, C. Howlett, J. R. Lucey, K. Said, T. M. Davis, J. Aguilar, S. Ahlen, A. J. Amsellem, J. Bautista, S. BenZvi, D. Bianchi, C. Blake, D. Brooks, A. Carr, T. Claybaugh, A. Cuceu, A. de la Maco

Published 2026-08-24

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: C. E. Ross, C. Howlett, J. R. Lucey, K. Said, T. M. Davis, J. Aguilar, S. Ahlen, A. J. Amsellem, J. Bautista, S. BenZvi, D. Bianchi, C. Blake, D. Brooks, A. Carr, T. Claybaugh, A. Cuceu, A. de la Macorra, B. Dey, P. Doel, K. Douglass, S. Ferraro, A. Font-Ribera, J. E. Forero-Romero, E. Gaztañaga, S. Gontcho A Gontcho, G. Gutierrez, J. Guy, K. Honscheid, D. Huterer, M. Ishak, R. Joyce, A. G. Kim, A. Kremin, O. Lahav, C. Lamman, M. Landriau, L. Le Guillou, A. Leauthaud, M. E. Levi, P. Martini, A. Meisner, R. Miquel, J. Moustakas, A. Muñoz-Gutiérrez, S. Nadathur, N. Palanque-Delabrouille, W. J. Percival, C. Poppett, F. Prada, I. Pérez-Ràfols, F. Qin, G. Rossi, E. Sanchez, D. Schlegel, M. Schubnell, D. Sprayberry, G. Tarlé, R. J. Turner, B. A. Weaver, R. Zhou, H. 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 expanding, a fact established for nearly a century. But the rate of that expansion is not the same everywhere, nor is it perfectly smooth. Galaxies do not merely drift apart with the cosmic flow; they also have their own local motions, tugged by the gravity of nearby clusters and pushed away by the voids between them. These local movements are called peculiar velocities. By measuring how fast a galaxy is moving toward or away from us, beyond the general expansion, astronomers can map the invisible web of dark matter that shapes our cosmic neighborhood. This map is crucial because it helps scientists test whether our current understanding of the universe, including the mysterious force driving its accelerated expansion known as dark energy, is correct. To do this, researchers need a way to measure both the true distance to a galaxy and its speed. While speed is relatively easy to determine from the color of light, finding the true distance is notoriously difficult, requiring a reliable "ruler" that works across vast stretches of space.

A team of astronomers has now created the largest single collection of these peculiar velocity measurements ever assembled, using data from the Dark Energy Spectroscopic Instrument, or DESI. This instrument, mounted on a telescope in Arizona, captures the light from millions of galaxies. The researchers focused on a specific type of galaxy known as an early-type galaxy, which are generally old, red, and shaped like smooth ellipsoids. For these galaxies, there is a reliable relationship between three observable properties: how fast the stars inside are moving in random directions, how bright the galaxy appears on average, and how large it looks in the sky. This relationship, known as the Fundamental Plane, acts as a cosmic ruler. Because the internal speed of the stars and the average brightness do not change based on how far away the galaxy is, astronomers can use them to calculate the galaxy's true physical size. By comparing this true size to how large the galaxy appears to us, they can determine exactly how far away it is.

The paper presents a catalogue containing these distance and velocity measurements for 98,292 unique galaxies. This number represents a doubling of all the Fundamental Plane distance measurements that have ever been recorded for galaxies in our local universe. The researchers achieved this by carefully selecting galaxies from the first year of DESI operations, filtering out stars and other objects that would confuse the measurements, and ensuring the light from each galaxy was captured with high precision. They then measured the speed of the stars within each galaxy and combined this with the galaxy's brightness and size to calculate its distance. The process involved rigorous checks to ensure the data was clean, removing any galaxies where the measurements were uncertain or where the light might have been distorted by dust or overlapping images. The final result is a dataset where the distance to each galaxy is known with an uncertainty of about 26 percent, a level of precision comparable to previous, much smaller surveys.

One of the most significant aspects of this work is the sheer scale and homogeneity of the sample. Previous surveys often combined data from many different telescopes and methods, which could introduce subtle inconsistencies. In contrast, this entire catalogue comes from a single instrument using a single method, which reduces the chance of hidden errors. The researchers also checked for systematic issues, such as whether the measurements changed depending on where in the sky the galaxy was located or whether the galaxy was observed during bright moonlight versus dark nights. They found that their measurements were consistent across the sky and that any small differences between observation times could be corrected. They also compared their results with an existing, comprehensive catalogue of galaxy distances called CosmicFlows-4 and found that the two sets of data agreed well, further validating their approach.

The resulting map of peculiar velocities shows that the local universe is not static. The measurements reveal a slight overall flow of galaxies moving in a specific direction, consistent with the gravitational pull of massive structures nearby. While the individual measurements of velocity for a single galaxy carry a large margin of error, the sheer number of galaxies in the catalogue allows scientists to see the big picture clearly. The data confirms that there is no strange acceleration or deceleration in the flow of galaxies as one looks further out, which supports current models of how the universe expands. This new catalogue serves as a foundation for a series of companion studies that will use these distances to measure the expansion rate of the universe and the growth rate of cosmic structures with unprecedented accuracy. By providing a massive, consistent set of local distance measurements, this work offers a powerful new tool to test the fundamental laws governing the cosmos.

Technical Summary: The DESI DR1 Peculiar Velocity Survey: Fundamental Plane Catalogue

Problem and Context
Measurements of peculiar velocities (PVs) in the local Universe ( $z < 0.1$ ) serve as a critical tool for studying dark energy and dark matter, offering a method to break the degeneracy between observed and cosmological redshifts. While previous surveys (e.g., CosmicFlows-4) have provided valuable constraints on the local expansion rate ( $H_0$ ) and structure growth rate ( $f\sigma_8$ ), they often rely on combining heterogeneous datasets from independent surveys using different tracers (Fundamental Plane vs. Tully-Fisher) and instruments. This approach introduces systematic uncertainties and discrepancies in zero-pointing and calibration. The Dark Energy Spectroscopic Instrument (DESI) aims to address these issues by creating a large, homogeneous PV catalogue using a single instrument and photometric system. This paper presents the first major release of the DESI Fundamental Plane (FP) PV catalogue, extending the work of the Early Data Release (EDR) to a significantly larger sample.

Methodology
The authors constructed the catalogue by extracting a sample of early-type galaxies from the DESI Data Release 1 (DR1) and the DESI Legacy Imaging Surveys (DR9). The methodology involved several rigorous stages:

Selection Criteria: The initial sample was drawn from the DESI Bright Galaxy Survey (BGS) and specific PV targets. Strict cuts were applied to ensure high-quality data:
- Spectroscopic classification as galaxies with robust redshifts ( $ZWARN=0$ , $\Delta\chi^2 > 30$ ).
- Redshift range $0.0033 < z < 0.1$ to balance peculiar velocity signal against measurement errors.
- Magnitude limits ( $10.0 < m_r < 18.0$ ) and color cuts to isolate red, early-type galaxies.
- Morphological cuts requiring de Vaucouleurs profiles or Sérsic indices $n_s > 2.5$ and axial ratios $b/a > 0.3$ .
- Velocity dispersion ( $\sigma_v$ ) cuts ( $50 < \sigma_v < 420$ km s $^{-1}$ ) to ensure reliable measurements.
Photometric and Spectroscopic Consistency:
- The authors investigated magnitude offsets between the DECaLS and BASS/MzLS surveys. An internal comparison using a population model revealed an offset of $-0.018 \pm 0.013$ mag, consistent with zero; thus, no magnitude correction was applied.
- Velocity dispersions were measured using Penalised Pixel-Fitting (pPXF) on the DESI blue-band spectra (3600–5550 Å).
- Internal consistency checks compared bright-time and dark-time observations, revealing a systematic offset where dark-time measurements yielded higher dispersions. A correction of $+2.3 \pm 0.1$ km s $^{-1}$ was applied to bright-time data.
- External consistency checks against SDSS DR16 (Portsmouth catalogue) showed good agreement within 5% of errors, with a slight underestimation of errors in both datasets that was corrected by inflating DESI velocity dispersion errors by a factor of 1.13.
Fundamental Plane Fitting:
- The authors fitted a 3D Gaussian Fundamental Plane relation ( $\log R_e = a \log \sigma_0 + b \log I_e + c$ ) using a maximum likelihood method that accounts for selection biases and intrinsic scatter.
- Aperture corrections ( $\alpha_{ap} = -0.06 \pm 0.03$ ) and evolution corrections ( $Q = 1.1 \pm 0.4$ ) were applied to standardize the parameters.
- A "group richness" correction was implemented to address a bias where galaxies in richer groups exhibited more negative log-distance ratios, a trend consistent with SDSS findings.
- The zero-point was anchored using independent distance indicators (Pantheon+SH0ES supernovae and Tully-Fisher data) in a companion paper (Carr et al., 2025), allowing for the derivation of $H_0$ .
Derivation of Peculiar Velocities:
- Log-distance ratios ( $\eta$ ) were calculated by comparing observed effective radii to those predicted by the FP relation.
- Peculiar velocities were derived from $\eta$ using the Carreres et al. (2023) estimator, converting the log-distance ratio into a velocity in the CMB frame.

Key Contributions and Results

Catalogue Size: The paper releases a catalogue of 98,292 unique early-type galaxies with FP-based distances and peculiar velocities. This increases the total number of measured $z < 0.1$ FP distances by a factor of $\sim 2$ compared to previous works.
Precision: The distance measurements achieve a random error precision of 26%, comparable to previous major surveys (e.g., SDSS FP).
Systematics Control: The study provides extensive systematics checks, including:
- Validation of photometric consistency between survey footprints.
- Correction for bright/dark time observation biases.
- Identification and correction of group-richness bias in log-distance ratios.
- Demonstration that the FP relation is robust against variations in selection criteria (e.g., Sérsic index, velocity dispersion cuts), though tighter cuts on star formation indicators (H $\alpha$ EW) suggest minor spiral contamination remains.
Clustering Subsample: A subset of 73,822 galaxies was curated specifically for cosmological clustering analyses, restricted to the DESI BGS footprint and excluding very low-redshift galaxies ( $z < 0.01$ ) to minimize estimator biases.

Significance
The paper claims that this catalogue represents the largest single set of peculiar velocity measurements to date. By utilizing a single instrument (DESI) and a homogeneous selection process, the survey simplifies the combination of datasets and reduces sources of systematic discrepancy inherent in multi-survey compilations. The authors note that while the DR1 sample covers only a fraction of the final survey area, the current success rate suggests the full survey will yield approximately 100,000 additional FP galaxies, exceeding initial projections. This large, homogeneous dataset is positioned to be transformative for "cosmic cartography" and for testing cosmological models through precise measurements of $H_0$ and $f\sigma_8$ at low redshifts, as demonstrated in the accompanying series of papers. The work establishes a robust pipeline for future data releases (DR2 and beyond) where the sample will be even more complete and the systematic uncertainties further reduced.

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