The Cosmic Web in the DESI Early Data Release: A Probabilistic Environment Catalog

Abstract

We present the first public cosmic-web environment catalog built on any DESI data release. Using ASTRA (Algorithm for Stochastic Topological RAnking), we classify each object in the DESI Early Data Release into void, sheet, filament, or knot by combining observed positions with matched random catalogs, without reconstructing a continuous density field. We apply this method to four DESI extragalactic tracers (BGS, LRG, ELG, and QSO) across the 20 EDR rosettes ($\sim 175$ deg$^2$ total), running 100 realizations per tracer-zone pair to derive per-object membership probabilities and classification entropies. We calibrate the classification thresholds using BGS as an anchor to match the volume-filling fractions reported for GAMA, and recover a physically consistent web morphology across all tracers. For BGS, the resulting web-type fractions and the environmental dependence of star formation are consistent with GAMA, COSMOS, and SDSS-based references, validating the method against established benchmarks. A normalized mutual information analysis on BGS reveals a clear dependence of the statistical associations between galaxy color, stellar mass, and specific star formation rate across environments. These results provide a new observational baseline for galaxy evolution studies with DESI. All data products and the open-source pipeline are publicly available.

Figures (14)

• Figure 1: Redshift distribution of extragalactic objects in the DESI EDR, considering the full sample of the 20 zones (rosettes) used in this work. The numbers of objects are 241,746, 112,649, 267,345, and 35,566 for BGS, LRG, ELG, and QSO, respectively.
• Figure 2: Example of the Delaunay tessellation built on the combined object and random catalog, $\mathcal{M}\equiv\mathcal{O}\cup\mathcal{R}$. Circles represent data points, triangles random points, and colors indicate the corresponding $r$ values.
• Figure 3: Cumulative distribution function of $r$ for the real and random catalogs, separated by DESI tracer. Lines show the average over 100 realizations; shaded bands show the $1\sigma$ dispersion across the 20 zones.
• Figure 4: Polar RA--$z$ distribution for the four extragalactic tracers across the 20 zones, colored by $r \in [-1,1]$. The inset ($\mathrm{RA} = 145^\circ$--$275^\circ$, $z = 0$--$0.1$) highlights filaments and knots in reddish tones ($r > 0$) and voids and sheets in bluish tones ($r < 0$).
• Figure 5: Cone diagrams for zone 0, showing the most probable ASTRA classification for each real object across the four tracers.