The DESI DR1 Peculiar Velocity Survey: Fundamental Plane Catalogue

TL;DR

The DESI DR1 Fundamental Plane peculiar velocity catalogue provides the largest $z<0.1$ PV dataset to date by leveraging a homogeneous FP analysis on a vast DESI DR1 sample. It combines robust photometric/spectroscopic selection, careful treatment of velocity dispersions, and a maximum-likelihood FP fit to derive log-distance ratios and peculiar velocities while marginalizing over key systematics such as aperture and evolution corrections. The zero-point of the FP is anchored via a joint FP+TF and Pantheon+SH0ES framework, enabling direct constraints on $H_0$ and the growth rate with DESI data. The resulting catalogue includes $98{,}292$ FP distances and $73{,}822$ high-quality PVs for clustering analyses, representing a substantial advance in low-redshift cosmography and setting the stage for companion DESI papers on $H_0$ and $f \sigma_8$ measurements. The data products, together with mock catalogues, will support cross-survey comparisons and guide improvements in later DESI data releases.

Abstract

Measurements of peculiar velocities in the local Universe are a powerful tool to study the nature of dark energy at low ($z < 0.1$) redshifts. Here we present the largest single set of $z<0.1$ peculiar velocity measurements to date, obtained using the Fundamental Plane (FP) of galaxies in the first data release (DR1) of the Dark Energy Spectroscopic Instrument (DESI). We describe the photometric and spectroscopic selection criteria used to define the sample, as well as extensive quality control checks on the photometry and velocity dispersion measurements. Additionally, we perform detailed systematics checks for the many analysis parameters in our pipeline. Our DESI DR1 catalogue contains FP-based distances and peculiar velocities for $98,292$ unique early-type galaxies, increasing the total number of $z < 0.1$ FP distances ever measured by a factor of $\sim2$. We achieve a precision of $26\%$ random error in our distance measurements which is comparable to previous surveys. A series of companion DESI papers use the distances and peculiar velocities presented in this paper to measure cosmological parameters.

Figures (17)

• Figure 1: Apparent magnitudes and best-fit truncated Gaussian distribution for the DECaLS (orange) and BASS/MzLS (blue) 'primary' subsets of the FP sample with magnitude cuts at $13<m_r<18$ mag.
• Figure 2: An example spectrum with velocity dispersion fit using pPXF. The plot shows the blue camera spectrum for the DESI targetid 39628467234408528. The spectrum is plotted as relative flux as a function of observation-frame wavelength, with 'relative flux' referring to the observed flux normalised to the median flux of the spectrum. The observed spectrum is plotted in black with the best fit pPXF model plotted in red. In green and blue we show the residuals with the blue points indicating regions of the spectrum that were masked during the fit process as they contain known background emission wavelengths.
• Figure 3: Distribution of on-sky separation (in arcsec) between duplicate and primaryVdisp fibre locations for repeat observations of the same galaxy. The red dashed line is the on-sky fibre radius for DESI fibres. All but one duplicate observation overlaps with the primary observation.
• Figure 4: Variation of velocity dispersion measurements across the sky for cells with more then 20 observed galaxies. Each cell is colour coded based of the mean velocity dispersion of the cell which is then normalised to the global mean velocity dispersion.
• Figure 5: Relative velocity dispersion error vs SNR for both bright (orange) and dark time (purple) observations. The bright time observations are consistent with the early data release (EDR) relation observed in Said_EDR with the dark time observations on average obtaining tighter constraints on the velocity dispersions measurements compared to equivalent SNR bright time observations.