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arxiv: 2509.18086 · v2 · submitted 2025-09-22 · 🌌 astro-ph.CO · astro-ph.GA

DESI Strong Lens Foundry III: Keck Spectroscopy for Strong Lenses Discovered Using Residual Neural Networks

Pith reviewed 2026-05-18 14:10 UTC · model grok-4.3

classification 🌌 astro-ph.CO astro-ph.GA
keywords strong gravitational lensingsource redshiftsinfrared spectroscopyKeck NIRESDESIresidual neural networkslens modelinggravitational lenses
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The pith

NIRES infrared spectroscopy combined with DESI optical data yields complete redshifts for six strong lens systems discovered by residual neural networks.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper reports Keck NIRES infrared spectroscopic observations of strong lenses and their source galaxies. The observations target eight lensed sources whose redshifts are difficult to measure in optical light alone. Source redshifts are determined for six systems in the range from 1.675 to 3.332. When merged with optical spectroscopy from the DESI Strong Lensing Secondary Target Program, the work supplies complete lens and source redshifts for six systems. These redshifts support lensing modeling to extract physical parameters and help refine automated lens searches for future wide-field surveys.

Core claim

The central claim is that Keck NIRES infrared spectroscopy successfully measures source redshifts for six strong lens systems found via Residual Neural Networks in the DESI Legacy Imaging Surveys, with values between z_s = 1.675 and 3.332. Combined with DESI optical spectroscopy, this delivers full redshift information for both lenses and sources in six cases, while two non-detections are linked to shorter 600-second exposures at high airmass.

What carries the argument

Keck NIRES infrared echellette spectrometer targeting emission lines from lensed source galaxies at redshifts difficult to access optically.

If this is right

  • These redshifts enable extraction of physical parameters from detailed lensing modeling for the six systems.
  • The results supply a resource for refining automated strong lens searches in future deep- and wide-field imaging surveys.
  • The redshift data supports investigation of a range of questions in astrophysics and cosmology using strong lensing.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same NIRES-plus-DESI approach could be scaled to the much larger lens samples expected from surveys such as LSST or Euclid.
  • Confirmed high-redshift sources provide magnified views that could be used to study early galaxy properties in greater detail.
  • Success of residual neural networks on this sample offers a benchmark for training lens-finding algorithms on other large imaging datasets.

Load-bearing premise

The observed spectral features with NIRES correctly identify emission lines from the lensed source galaxies at the reported redshifts, and the two non-detections result only from shorter exposures at high airmass.

What would settle it

A longer-exposure or higher-sensitivity spectrum of either non-detected system that shows no emission lines at the wavelengths expected for the reported redshifts or reveals lines at a clearly different redshift.

Figures

Figures reproduced from arXiv: 2509.18086 by A. Bolton, A. Cikota, A. de la Macorra, A. D. Myers, A. Filipp, A. Lambert, A. Meisner, Arjun Dey, B. A. Weaver, Christopher J. Storfer, C. Poppett, D. Brooks, D.J. Schlegel, D. Sprayberry, E. Gazta\~naga, E. Jullo, E. Sanchez, E. Sukay, F. Prada, G. Aldering, G. Gutierrez, G. Rossi, G. Tarl\'e, H. Zou, I. P\'erez-R\`afols, J. Aguilar, J. E. Forero-Romero, J. Moustakas, K. Honscheid, K.J. Kwon, L. Le Guillou, Marcos Tamargo-Arizmendi, M. Ishak, M. Landriau, M. Schubnell, N. Suzuki, P. Doel, R. Kehoe, R. Miquel, S. Ahlen, S. BenZvi, S. E. Koposov, S. Gontcho A Gontcho, Shrihan Agarwal, S. Juneau, S. Perlmutter, Suchitoto Tabares-Tarquinio, T. Claybaugh, T. Kisner, William Sheu, Xiaosheng Huang, Y. Shu.

Figure 1
Figure 1. Figure 1: DESI J006.3643+10.1853. Left Column: The first and second rows show the spectra for each source image #1 and #2. The third row shows the weighted coaddition of the source image pair. The spectra in these panels are smoothed with a kernel of 10 pixels. The fourth row shows the zoom-in for individual emission lines (maroon), alongside the error spectrum (black), with a convolution kernel of 5 pixels. The err… view at source ↗
Figure 2
Figure 2. Figure 2: DESI J094.5639+50.3059. For the arrangement of the panels, see the caption of [PITH_FULL_IMAGE:figures/full_fig_p008_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: DESI J133.3800+23.3652. For the arrangement of the panels, see the caption of [PITH_FULL_IMAGE:figures/full_fig_p009_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: DESI J154.5307-00.1368. For the arrangement of the panels, see the caption of [PITH_FULL_IMAGE:figures/full_fig_p010_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: DESI J165.4754-06.0423. For the arrangement of the panels, see the caption of [PITH_FULL_IMAGE:figures/full_fig_p011_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: DESI J215.2654+00.3719. Left Column: In the top row, the left and right panels show the zoom-in for the coadded 1D emission lines [O III] and Hα (maroon), respectively, alongside the error spectrum (black), with a convolution kernel of width 2 pixels. The 2D spectra are presented in the bottom row, with vertical lines near the top showing the alignment with 1D emission lines. The locations of the source ar… view at source ↗
read the original abstract

We present spectroscopic data of strong lenses and their source galaxies using the Keck Near-Infrared Echellette Spectrometer (NIRES) and the Dark Energy Spectroscopic Instrument (DESI), providing redshifts necessary for nearly all strong-lensing applications with these systems, especially the extraction of physical parameters from lensing modeling. These strong lenses were found in the DESI Legacy Imaging Surveys using Residual Neural Networks (ResNet) and followed up by our Hubble Space Telescope program, with all systems displaying unambiguous lensed arcs. With NIRES, we target eight lensed sources at redshifts difficult to measure in the optical range and determine the source redshifts for six, between $z_s$ = 1.675 and 3.332. DESI observed one of the remaining source redshifts, as well as an additional source redshift within the six systems. The two systems with non-detections by NIRES were observed for a considerably shorter 600s at high airmass. Combining NIRES infrared spectroscopy with optical spectroscopy from our DESI Strong Lensing Secondary Target Program, these results provide the complete lens and source redshifts for six systems, a resource for refining automated strong lens searches in future deep- and wide-field imaging surveys and addressing a range of questions in astrophysics and cosmology.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript presents Keck NIRES near-infrared spectroscopy targeting eight lensed sources from strong lens systems discovered via Residual Neural Networks in the DESI Legacy Imaging Surveys. Source redshifts are successfully measured for six systems (zs = 1.675–3.332), with DESI providing one additional source redshift and optical data; combined with the DESI Strong Lensing Secondary Target Program, this yields complete lens and source redshifts for six systems.

Significance. If the redshift measurements are secure, the work supplies essential spectroscopic data for strong-lensing modeling, enabling extraction of physical parameters and supporting studies in astrophysics and cosmology. It also serves as a practical resource for refining automated lens searches in future wide-field surveys by demonstrating the utility of combined optical and near-IR follow-up for high-redshift sources.

major comments (2)
  1. NIRES Observations section: The attribution of the two non-detections solely to the shorter 600 s exposures at high airmass lacks supporting quantitative details such as noise estimates, achieved sensitivity, or expected emission-line flux limits; without these, alternative explanations (e.g., intrinsically faint lines) cannot be ruled out and weaken in the overall success rate.
  2. Results section on redshift determinations: The specific emission lines used to derive each of the six source redshifts, together with their observed wavelengths and any velocity offsets, are not listed or tabulated; this information is required to verify that the features are correctly attributed to the background lensed galaxies rather than sky residuals, lens-galaxy lines, or noise.
minor comments (2)
  1. Introduction: Include explicit cross-references to the earlier papers in the DESI Strong Lens Foundry series to clarify the progression from discovery (Paper I/II) to spectroscopic follow-up.
  2. Figure presentation: Ensure that any spectra figures include clear line identifications, wavelength scales, and error spectra so readers can directly assess the redshift assignments.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive comments and positive assessment of the manuscript. We address each major comment below and will incorporate the suggested improvements in the revised version.

read point-by-point responses
  1. Referee: NIRES Observations section: The attribution of the two non-detections solely to the shorter 600 s exposures at high airmass lacks supporting quantitative details such as noise estimates, achieved sensitivity, or expected emission-line flux limits; without these, alternative explanations (e.g., intrinsically faint lines) cannot be ruled out and weaken in the overall success rate.

    Authors: We agree that quantitative details would strengthen the discussion of the non-detections. In the revised manuscript we will add noise estimates, achieved sensitivity, and expected emission-line flux limits derived from the 600 s exposures, airmass, and NIRES instrument performance. These additions will help contextualize the success rate and address possible alternative explanations. revision: yes

  2. Referee: Results section on redshift determinations: The specific emission lines used to derive each of the six source redshifts, together with their observed wavelengths and any velocity offsets, are not listed or tabulated; this information is required to verify that the features are correctly attributed to the background lensed galaxies rather than sky residuals, lens-galaxy lines, or noise.

    Authors: We acknowledge the value of this information for verification. The revised manuscript will include a table listing the specific emission lines, observed wavelengths, and any velocity offsets used for each of the six source redshifts. This will allow readers to confirm the attributions to the lensed sources. revision: yes

Circularity Check

0 steps flagged

No circularity: direct observational redshift measurements

full rationale

This paper is a pure observational data report presenting Keck NIRES and DESI spectra for strong-lens systems discovered via ResNet. Redshifts are obtained by direct line identification against standard wavelength references with no equations, fitted parameters, derivations, or load-bearing self-citations. The six reported source redshifts and two non-detections are empirical results, not predictions or renamings that reduce to the paper's own inputs. The work is self-contained against external spectroscopic standards and contains no derivation chain to analyze.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The paper is an observational astronomy report presenting new data. It does not introduce free parameters, new axioms beyond standard practices, or invented entities.

axioms (1)
  • standard math Standard assumptions in astronomical spectroscopy for line identification and redshift calculation from observed wavelengths.
    Invoked implicitly when determining redshifts from spectral features in NIRES and DESI data.

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