The Power of DESI for Photometric Redshift Calibration: A Case Study with KiDS-1000

TL;DR

This study evaluates the Dark Energy Spectroscopic Instrument (DESI) as a favorable resource for photometric-redshift calibration in weak-lensing surveys, using KiDS-1000 and a Self-Organizing Map (SOM) approach to map color–magnitude space. By calibrating $n(z)$ via $p(z|c)$ within SOM cells and applying DESI and KiDS spectroscopic samples, the work finds broad agreement with KiDS-based redshift distributions but reveals small but systematic shifts in $\langle z\rangle$ and $\tilde{z}$ in tomographic edges, driven by spectroscopic incompleteness and DESI's redshift limit of $z<1.6$. The analysis shows that when restricted to jointly occupied SOM cells, DESI increases spectroscopic coverage by a factor of ~1.6 in overlapping regions, but differences in color–magnitude selection between surveys introduce biases in the high- and low-redshift tails, especially where zCOSMOS-D contributes high-$z$ sources. The results highlight the importance of uniform, deep photometric coverage to fully exploit DESI for photo-$z$ calibration and set the stage for extended footprints and future joint analyses with upcoming surveys like LSST and Euclid.

Abstract

Accurate redshift estimates are a critical requirement for weak lensing surveys and one of the main uncertainties in constraints on dark energy and large-scale cosmic structure. In this paper, we study the potential to calibrate photometric redshift (photo-z) distributions for gravitational lensing using the Dark Energy Spectroscopic Instrument (DESI). Since beginning its science operations in 2021, DESI has collected more than 50 million redshifts, adding about one million monthly. In addition to its large-scale structure samples, DESI has also acquired over 256k high-quality spectroscopic redshifts (spec-zs) in the COSMOS and XMM and VVDS fields. This is already a factor of 3 larger than previous spec-z calibration compilations in these two regions. Here, we explore calibrating photo-zs for the subset of KiDS-1000 galaxies that fall into joint self-organizing map (SOM) cells overlapping the DESI COSMOS footprint using the DESI COSMOS observations. Estimating the redshift distribution in KiDS-1000 with the new DESI data, we find broad consistency with previously published results while also detecting differences in the mean redshift in some tomographic bins with an average shifts of Delta Mean(z) = -0.028 in the mean and Delta Median(z) = +0.011 in the median across tomographic bins. However, we also find that incompleteness per SOM cell, i.e., groups of galaxies with similar colors and magnitudes, can modify n(z) distributions. Finally, we comment on the fact that larger photometric catalogs, aligned with the DESI COSMOS and DESI XMM and VVDS footprints, would be needed to fully exploit the DESI dataset and would extend the coverage to nearly eight times the area of existing 9-band photometry.

Figures (14)

• Figure 1: Results of a sanity test to ensure that our SOM code can reproduce the KiDS results. We display the published redshift distributions ($n(z)$) from the KiDS survey in black. Overlayed in purple are our recreated $n(z)$s, reconstructed using our SOM methodology and the original KiDS calibration data (no DESI data). Each panel corresponds to a specific tomographic bin, composed of a subset of galaxies within the specified redshift range between 0.1 $\leq Z_B \leq$ 1.2. The values for the mean ($\langle z \rangle$) and median ($\tilde{z}$) redshifts are listed for both the published KiDS data (S$_K$) and our reconstruction (S$_{RK}$).
• Figure 2: Spatial distribution of the DESI COSMOS spectra (purple), the KiDS COSMOS spectra (teal), and the KiDS deep photometry in the COSMOS field (black crosses). A random 0.5% subsample of each catalog is shown, yielding 117 DESI spec-zs, 50 KiDS spec-zs, and 502 KiDS deep photo-z galaxies within the shared region highlighted by the pink square, which is enlarged in the right-hand panel. The KiDS COSMOS samples occupy only a fraction of the DESI footprint in the COSMOS field. The KiDS-1000 deep photometric catalog does not overlap with the XMM/VVDS field and is therefore not shown. Within the inner shared area, DESI has a total of 19,866 spec-z’s, whereas KiDS has a total 9,930 spec-z’s.
• Figure 3: Distribution of the relative unweighed occupation of shared SOM cells, n(c$_i$), where each cell is jointly occupied by both the DESI and KiDS COSMOS spectroscopic redshifts. This ratio is defined as the number of DESI spec-zs divided by the number of KiDS spec-zs per shared cell. On average, DESI provides 1.6 more redshifts than KiDS, and this is denoted by the black vertical line.
• Figure 4: Spec-z occupation and relative joint completeness in SOM space. The left panel shows the occupation density of the original $S_D$ sample (i.e., all DESI targets that pass the Ratajczak2025 cuts), with colors representing the number of spectra in each SOM cell. Grey cells in the left panel correspond to regions unoccupied by $S_D$ (i.e., $n(z_D)=0$). The right panel shows the ratio of occupation between the joint $S_D$ and joint $S_K$ samples, where brighter colors indicate cells with a substantially higher count in DESI relative to KiDS. Grey cells in the right panel mark regions that are not jointly occupied by both surveys. The original $S_D$ sample covers 43.2% of the entire SOM map. The left figure only includes the joint SOM cells, which are occupied by both KiDS and DESI, collectively accounting for 26.9% of the entire SOM map.
• Figure 5: Spatial distribution of the mean r-band magnitude within each joint SOM cell for the KiDS and DESI spectroscopic samples. The left panel shows the mean r-band magnitude for KiDS, calculated using KiDS deep photometry. The central panel illustrates the corresponding mean r-band magnitude for DESI, also derived from KiDS deep photometry. The right panel represents the magnitude difference between KiDS and DESI, where positive values indicate regions where KiDS appears relatively brighter (or DESI appears fainter). Some regions in SOM space exhibit notably large magnitude differences in absolute values, with values reaching up to $\sim$0.6 magnitudes (highlighted in red or blue). Across all joint SOM cells, the average difference in $r$-band magnitude between DESI and KiDS is 0.24.