🔭 astrophysics

DESI Strong Lens Foundry IV: Spectroscopic Confirmation of DESI Lens Candidates with VLT/MUSE

This paper presents the spectroscopic confirmation of 55 strong gravitational lens candidates from the DESI Legacy Imaging Surveys using VLT/MUSE integral field observations, successfully determining redshifts for both lenses and sources to expand the sample for dark matter and cosmological studies.

Original authors: Emerald Lin, Ivonne Toro Bertolla, Aleksandar Cikota, Xiaosheng Huang, Christopher J. Storfer, Marcos Tamargo-Arizmendi, David J. Schlegel, William Sheu, Nao Suzuki

Published 2026-06-12

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Emerald Lin, Ivonne Toro Bertolla, Aleksandar Cikota, Xiaosheng Huang, Christopher J. Storfer, Marcos Tamargo-Arizmendi, David J. Schlegel, William Sheu, Nao Suzuki

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 Big Picture: Hunting for Cosmic Mirrors

Imagine the universe is a giant, dark room filled with billions of tiny flashlights (galaxies). Occasionally, a massive object, like a giant glass marble (a galaxy cluster), sits between you and a distant flashlight. Because of gravity, that marble bends the light, acting like a funhouse mirror. It can stretch the distant light into a long, glowing arc, or even split it into multiple copies of the same object.

This phenomenon is called strong gravitational lensing. It is a powerful tool for scientists because it lets them weigh invisible "dark matter" and study galaxies that are too far away to see otherwise.

However, finding these cosmic mirrors is like looking for a needle in a haystack. The "DESI Legacy Imaging Surveys" took a massive photo of the sky and used a computer brain (a neural network) to find about 3,500 potential candidates. But a photo isn't enough; you need to know exactly what those objects are and how far away they are to confirm they are real lenses.

The Mission: The "MUSE" Microscope

This paper describes the fourth step in a series of projects called the "DESI Strong Lens Foundry." The team took 76 of the most promising candidates found by the computer and pointed a very special telescope at them.

They used MUSE (Multi Unit Spectroscopic Explorer) on the Very Large Telescope in Chile. Think of MUSE not just as a camera, but as a prism-powered microscope.

The Camera: It takes a picture of the sky.
The Prism: It splits the light from every single pixel in that picture into a rainbow (a spectrum).

By looking at these rainbows, the team can measure the "redshift" of the objects. Redshift is like the Doppler effect for light: just as a siren sounds lower-pitched as an ambulance drives away, light from a galaxy moving away from us shifts toward the red end of the spectrum. The redder the light, the farther away the galaxy is.

To confirm a lens, you need two things:

A Lens Galaxy (the marble) in the foreground.
A Source Galaxy (the distant flashlight) in the background.
They must be at different distances. If they are at the same distance, they aren't a lens; they are just neighbors.

What They Did

The team observed 76 targets between 2022 and 2024. They extracted the light from 223 different objects (the lenses, the sources, and random stars or galaxies that happened to be in the way).

The Results:

55 Confirmed Lenses: For these systems, they successfully measured the distance to both the lens and the source. They confirmed these are real cosmic mirrors. Some of these are complex, like a single background galaxy split into four images (an "Einstein Cross") or a long arc stretched around a group of galaxies.
15 "Lens Only" Systems: For these, they could measure the distance to the foreground galaxy (the lens), but the background source was too faint or the weather was too bad to get a clear reading. They know a lens is likely there, but they need more data to be sure about the background object.
6 False Alarms: These looked like lenses in the photos, but when the team looked at the light spectra, they realized the "arc" was actually just a spiral arm of the same galaxy, or two galaxies that were just sitting next to each other by chance. They are not gravitational lenses.

Why This Matters (According to the Paper)

The paper explains that this work is like building a verified library of cosmic mirrors.

Complex Configurations: They found systems with multiple background sources or complex groups of galaxies, which are hard to study with other telescopes.
Southern Hemisphere: Many of these targets are in the southern sky, where other telescopes can't reach easily. MUSE is one of the few tools that can see them clearly.
High-Redshift Sources: They found some very distant galaxies (high redshift) that are too far for other instruments to see their specific light features.

The Takeaway

The authors successfully turned a list of computer-generated guesses into a confirmed list of 55 real gravitational lenses. They used the unique ability of the MUSE instrument to look at the "rainbow" of light from many objects at once to prove that these cosmic mirrors exist. This confirmed list is now a valuable resource for other scientists who want to study dark matter and the structure of the universe, but the paper itself focuses strictly on the confirmation of these specific 76 targets and the data they collected.

Technical Summary: DESI Strong Lens Foundry IV: Spectroscopic Confirmation of DESI Lens Candidates with VLT/MUSE

Problem and Context
Strong gravitational lensing systems are critical tools for astrophysics and cosmology, offering unique capabilities to map dark matter distributions, detect low-mass halos, and constrain the Hubble constant ( $H_0$ ) via time-delay measurements. While the DESI Legacy Imaging Surveys have identified approximately 3,500 new strong lens candidates using residual neural networks (ResNet) and autocorrelation algorithms, a significant bottleneck remains: spectroscopic confirmation. Many candidates, particularly those in the southern hemisphere or with high-redshift sources, lack the necessary redshift data to be utilized for scientific studies. Existing facilities often struggle with the sensitivity required to detect weak spectral features in these challenging systems, particularly for sources beyond $z \sim 1.5$ where typical emission lines shift out of optical ranges.

Methodology
This study presents the spectroscopic confirmation of 76 DESI lens candidates using the Multi Unit Spectroscopic Explorer (MUSE) on the ESO Very Large Telescope (VLT). The observations were conducted between 2022 and 2024 across five ESO filler programs.

Instrumentation: MUSE was utilized in Wide Field Mode ( $60'' \times 60''$ field of view, $0.2''$ spatial sampling) covering the spectral range of 4750–9350 Å with a resolution of $R = 2000–4000$ .
Data Reduction: Observations consisted of four 700-second exposures per target. Data were reduced using the MUSE pipeline (v2.2) and the Zurich Atmosphere Purge (ZAP) tool for sky subtraction.
Analysis Strategy: Synthetic V, R, and I band images were generated from the data cubes to identify lens configurations. Spectra were extracted using circular apertures or manually selected spaxels for non-circular targets (e.g., arcs). Redshifts were determined by visually inspecting extracted 1D spectra and fitting Gaussian models to prominent features.
- Lens Identification: Primarily based on Ca II H & K absorption lines, the G-band, and other passive galaxy features.
- Source Identification: Dependent on redshift, features included the [O II] doublet ( $\lambda\lambda 3726, 3729$ ), metal absorption lines (e.g., Si II, C IV, Fe II, Mg II), and emission lines (e.g., [C III], Ly $\alpha$ ).
Quality Control: A quality flag ( $Q_z$ ) was assigned to each redshift: 1 (Robust), 2 (Probable), or 3 (Possible), based on signal-to-noise ratio and feature unambiguity.

Key Contributions and Results
The paper reports the analysis of 223 extracted spectra from 76 unique targets. The outcomes are categorized as follows:

Confirmed Systems (55 targets): Both lens and source redshifts were successfully determined for 55 systems. These include complex configurations such as multiple source planes, group/cluster-scale lenses, and Einstein crosses.
- Notable Findings: The sample includes high-redshift sources (e.g., $z \approx 3.46$ for DESI J168.7680+16.7604 Source B) and systems with multiple distinct background sources (e.g., DESI J090.9854-35.9683 with seven lensed images).
- Spectral Diversity: The study successfully identified sources using a wide array of features, including rare detections of Ly $\alpha$ emission and strong metal absorption lines in high-redshift galaxies.
Lens-Only Systems (15 targets): For 15 targets, lens redshifts were measured, but source redshifts could not be determined due to insufficient signal-to-noise or the absence of the source in the MUSE cubes (often due to weather conditions).
Confirmed Non-Lenses (6 targets): Six candidates were ruled out as gravitational lenses. Spectroscopic analysis revealed that the "arcs" were either tidal tails from merging galaxies, spiral arms, or foreground/background objects with redshifts consistent with the central galaxy (e.g., DESI J189.9885+12.6693 and DESI J333.3655-13.2491).

Significance and Claims
The authors position this work as a vital component of the "DESI Strong Lens Foundry" series, complementing HST imaging (Paper I), DESI spectroscopy (Paper II), and Keck near-IR follow-up (Paper III).

Southern Hemisphere Advantage: The paper emphasizes MUSE's unique capability to characterize southern hemisphere lenses where alternative spectroscopic facilities are limited, successfully pushing redshift determinations beyond $z \sim 1.5$ even in challenging cases.
Legacy Value: The resulting spectroscopically confirmed sample provides a robust dataset for future studies on dark matter substructure, galaxy evolution, and cosmology.
Methodological Validation: The work demonstrates the efficiency of combining machine learning discovery with targeted integral field spectroscopy (IFS) for follow-up, establishing a workflow for handling complex lensing environments (e.g., multiple source planes and interlopers) that are difficult to characterize with slit spectroscopy.

The paper concludes that this sample, combined with future high-resolution imaging (e.g., JWST, Euclid) and lens modeling, will enable precise measurements of dark matter halo profiles and independent constraints on the Hubble constant. The authors explicitly state that the strategies developed here lay the groundwork for future strong lens discoveries as wide-field surveys (LSST/Rubin, Roman) increase the demand for flexible, high-throughput follow-up.

The Big Picture: Hunting for Cosmic Mirrors

The Mission: The "MUSE" Microscope

What They Did

Why This Matters (According to the Paper)

The Takeaway

More like this