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arxiv: 2606.12613 · v1 · pith:T5BLZWC6new · submitted 2026-06-10 · 🌌 astro-ph.CO

Weak Lensing Spectrotomography: A1767 and A2065

Jubee Sohn (Seoul National University) , Ian P. Dell'Antonio (Brown University) , Margaret J. Geller (Smithsonian Astrophysical Observatory) This is my paper

Pith reviewed 2026-06-27 08:19 UTC · model grok-4.3

classification 🌌 astro-ph.CO
keywords weak lensingspectrotomographygalaxy clusterstomographic shearspectroscopic redshiftsA1767A2065
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The pith

Spectroscopic redshifts enable tomographic weak lensing detections in clusters A1767 and A2065 at 3.1 and 3.5 sigma.

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

The paper demonstrates the first use of spectrotomography on archival Subaru/HSC imaging for two galaxy clusters. Spectroscopic redshifts separate background galaxies cleanly from cluster members and suppress systematics that photometric redshifts can introduce. Tomographic shear signals are detected at 3.1 sigma in A1767 and 3.5 sigma in A2065. The measured amplitudes match independent dynamical caustic masses and increase with source redshift as expected. The work positions the method for wider use in unbiased cluster mass calibration and a potential geometric cosmological test.

Core claim

Spectrotomography using only galaxies with spectroscopic redshifts yields detections of tomographic shear signals at 3.1 sigma for A1767 and 3.5 sigma for A2065. The shear amplitudes are consistent with the clusters' dynamical caustic masses and scale appropriately with source redshift. This spectroscopic approach cleanly separates cluster members from the background and reduces potential systematics from photometric redshifts.

What carries the argument

Spectrotomography, weak lensing tomography that relies exclusively on spectroscopically confirmed background galaxies to separate them from cluster members.

If this is right

  • Shear signal amplitudes match the cluster dynamical caustic masses.
  • Signals scale appropriately with source redshift.
  • The spectroscopic method suppresses systematics that photometric redshifts can introduce.
  • The three detections including A2029 set the stage for broader unbiased cluster mass determinations and a potential geometric cosmological test.

Where Pith is reading between the lines

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

  • Subtle potential systematics noted in the A2029 comparison could affect applications to larger Subaru, Euclid, and LSST samples.
  • The technique opens a path to a geometric cosmological test independent of other probes once more clusters are measured.
  • Extension to additional clusters would provide a larger sample of unbiased weak lensing masses.

Load-bearing premise

Spectroscopic redshifts cleanly separate background galaxies from cluster members with negligible contamination or selection effects, and shear measurements on the bright background galaxies are free from subtle systematics.

What would settle it

A measurement in which the tomographic shear amplitudes fail to match the caustic dynamical masses or do not increase with source redshift would falsify the detection claim.

Figures

Figures reproduced from arXiv: 2606.12613 by Ian P. Dell'Antonio (Brown University), Jubee Sohn (Seoul National University), Margaret J. Geller (Smithsonian Astrophysical Observatory).

Figure 1
Figure 1. Figure 1: Ellipticity distributions for point sources in (a) A1767 and (b) A2065 fields. Black and yellow squares show raw ellipticities and ellipticities after the PSF correction, respectively. and e2 = 2Ixy Ixx + Iyy . (2) We then compute the PSF model ellipticities for each object, as determined by hscpipe. The yellow squares display the residuals for point-like sources. After the PSF correction, the mean ellipti… view at source ↗
Figure 2
Figure 2. Figure 2: Spectroscopic survey completeness for galaxies within the R200 from the center of (a) A1767 and (b) A2065 as a function of r−band magnitude, respectively. sight elongation due to the finger-of-god effect and transverse elongation due to the infall region. More importantly, the cone diagrams reveal voids and large-scale structures along the line of sight. At z ≳ 0.5, these structures become poorly defined a… view at source ↗
Figure 3
Figure 3. Figure 3: Cone diagrams for (upper) the A1767 field and (lower) the A2065 field. Black dots are galaxies with spectroscopic redshifts, and red dots mark the spectroscopic members of the two clusters. 0.0 0.2 0.4 0.6 0.8 redshift 100 200 300 400 N A1767, z = 0.071 Nspe (Rproj < R200, z > 0.1) = 1014 0.0 0.2 0.4 0.6 0.8 redshift A2065, z = 0.073 Nspe (Rproj < R200, z > 0.1) = 1621 [PITH_FULL_IMAGE:figures/full_fig_p0… view at source ↗
Figure 4
Figure 4. Figure 4: Redshift distribution for (a) the A1767 field and (b) the A2065 field. The red histogram highlights cluster members. the PSF subtraction, shape measurements, and intrinsic align￾ments. Second, the distributions of tangential (i.e., +, E-mode) and cross (i.e., ×, B-mode) components for the cluster mem￾bers should be identical because their shapes are not affected by the gravitational field of the cluster. T… view at source ↗
Figure 5
Figure 5. Figure 5: (a) The distributions of e1 (the black filled) and e2 (the red hatched) of spectroscopically identified members within R200 from the A1767 center. (b) The normalized difference between e1 and e2 distributions. (c-d) Same as (a-b), but for A2065 member galaxies. Azimuthal symmetry around the center of each cluster should produce indistinguishable distributions of the elliptici￾ties e1 and e2 for both cluste… view at source ↗
Figure 7
Figure 7. Figure 7: compares the raw and normalized distributions of etan (the red hatched) and ecross (the black filled) for mem￾bers of A1767 and A2065 within R200 of the cluster center. There is no significant difference between the etan and ecross distributions for members in either cluster, consistent with the expectation that cluster members do not experience lensing distortion from the cluster potential. The KS test p−… view at source ↗
Figure 8
Figure 8. Figure 8: shows the distributions of etan (red hatched his￾tograms) and ecross (black filled histograms) for background galaxies within R200 from the centers of A1767 and A2065, respectively. The distributions of etan for background galax￾ies are skewed distribution toward positive ellipticities. The low p−values (i.e., 0.03 for A1767 and 0.01 for A2065) from the K-S tests also indicate that the null hypothesis that… view at source ↗
Figure 9
Figure 9. Figure 9: Weak lensing significance (κ) maps for A1767 (left two panels) and A2065 (right two panels). For each cluster, the left panel shows the significance map constructed from spectroscopically identified background galaxies only, while the right panel shows the same map constructed from the full photometric background galaxy sample. show ⟨ecross⟩ (the B-mode, black crosses). For sources with z > 0.1, the B-mode… view at source ↗
Figure 10
Figure 10. Figure 10: Spectrotomographic signal as a function of redshift within R200 of the projected distance from the centers of (a) A1767 and (b) A2065. Red and black symbols show the tangential and cross ellipticities, respectively. The error bar indicates the 1σ standard deviation divided by √ Nbin, where Nbin is the number of galaxies in each redshift bin. Blue curves display the predicted ⟨etan⟩ we derived based on the… view at source ↗
Figure 11
Figure 11. Figure 11: Mean distributions of ellipticities (e1, e2, et, ec) of spectroscopically identified background (z > 0.1) galaxies as a function of clustercentric distance. Blue squares and red circles display galaxies in the A1767 and A2065 field, respectively. weak lensing maps, the detections based on background galax￾ies with spectroscopic redshifts are a step toward the ultimate reduction of systematic errors due to… view at source ↗
read the original abstract

We describe spectroscopic tomographic weak lensing measurements (spectrotomography) for two rich clusters of galaxies, A1767 and A2065, based on extensive spectroscopy and Subaru/Hyper Suprime-Cam (HSC) imaging. These detections represent the first use of spectrotomography based on archival Subaru/HSC imaging. The measurements depend only on galaxies with spectroscopic redshifts reported here. The approach cleanly separates cluster members from the background and suppresses systematics that may be introduced by the use of photometric background redshifts. We detect the tomographic shear signals at $3.1\sigma$ (A1767) and $3.5\sigma$ (A2065). The shear signal amplitudes are consistent with the cluster dynamical (caustic) masses and they scale appropriately with source redshift. However, comparison with the first spectrophotometric detection of A2029 based on DECam imaging reveals some subtle potential systematic issues in deriving the shear signal for the relatively bright background galaxies used in the analysis. These issues may be important for understanding future more extensive applications of spectrotomography based on further Subaru imaging, as well as Euclid and LSST data. The total of three spectrophotometric detections (A1767, A2029, and A2065) sets the stage for broader application of the technique for unbiased cluster weak lensing mass determinations and potentially for a geometric cosmological test that is independent of other methods.

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 the first application of spectroscopic tomographic weak lensing (spectrotomography) to clusters A1767 and A2065 using archival Subaru/HSC imaging combined with new spectroscopy. It reports tomographic shear detections at 3.1σ (A1767) and 3.5σ (A2065) based exclusively on galaxies with spectroscopic redshifts, claims consistency of shear amplitudes with independent dynamical (caustic) masses, and notes appropriate scaling with source redshift. The work also compares to a prior A2029 detection and flags potential subtle systematics in shear measurements for relatively bright background galaxies.

Significance. If the shear measurements prove robust, the result demonstrates a data-driven method for cluster weak-lensing masses that avoids photometric-redshift contamination and selection biases. The reported consistency with caustic masses and redshift scaling would provide independent validation. With three clusters now measured (including A2029), the technique is positioned for broader use in unbiased mass calibration and potentially for geometric cosmological tests independent of other probes, with direct relevance to Euclid and LSST data.

major comments (2)
  1. [Abstract] Abstract and final paragraph: the central claim of 3.1σ and 3.5σ detections rests on the assertion that shear amplitudes for the bright spectroscopic background sample are free of the subtle systematics identified in the A2029 DECam comparison. No quantitative test (null test, PSF-model variation, or magnitude-dependent residual analysis) is presented to show that any such bias is smaller than the reported significance; because the sample is restricted to spectroscopically confirmed objects, any measurement bias directly affects both the detection significance and the redshift-scaling test.
  2. [Methods (spectroscopic selection)] The assumption that spectroscopic redshifts yield negligible cluster-member contamination and no selection-induced bias in the shear sample is load-bearing for the mass-consistency and redshift-scaling statements, yet no explicit contamination fraction, purity estimate, or test against alternative redshift cuts is provided.
minor comments (2)
  1. [Abstract] The abstract states that signals 'scale appropriately with source redshift' but does not specify the number of tomographic bins or the exact redshift ranges used; adding this detail would clarify the tomographic aspect.
  2. Figure captions and text should explicitly state the magnitude limit and typical S/N of the spectroscopically selected background galaxies to allow readers to assess the relevance of the A2029 systematics warning.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful and constructive review of our manuscript. We address the major comments point by point below and will revise the manuscript to incorporate the suggested improvements.

read point-by-point responses

  1. Referee: [Abstract] Abstract and final paragraph: the central claim of 3.1σ and 3.5σ detections rests on the assertion that shear amplitudes for the bright spectroscopic background sample are free of the subtle systematics identified in the A2029 DECam comparison. No quantitative test (null test, PSF-model variation, or magnitude-dependent residual analysis) is presented to show that any such bias is smaller than the reported significance; because the sample is restricted to spectroscopically confirmed objects, any measurement bias directly affects both the detection significance and the redshift-scaling test.

    Authors: We agree that the absence of explicit quantitative tests for these clusters leaves the robustness of the 3.1σ and 3.5σ claims open to the concern raised. The manuscript already flags potential subtle systematics from the A2029 comparison, but we did not perform the specific null tests, PSF variations, or magnitude-dependent analyses for A1767 and A2065. We will add these tests in the revised manuscript to the extent permitted by the data, along with a clearer discussion of how any residual bias would impact the significance and scaling results. revision: yes

  2. Referee: [Methods (spectroscopic selection)] The assumption that spectroscopic redshifts yield negligible cluster-member contamination and no selection-induced bias in the shear sample is load-bearing for the mass-consistency and redshift-scaling statements, yet no explicit contamination fraction, purity estimate, or test against alternative redshift cuts is provided.

    Authors: While the use of spectroscopic redshifts provides a clean separation in principle, we acknowledge that the manuscript lacks the explicit quantitative estimates and sensitivity tests requested. We will revise the methods section to include contamination fraction and purity estimates derived from the spectroscopic catalog, as well as tests of the shear signal under alternative redshift cuts, to better support the mass-consistency and scaling claims. revision: yes

Circularity Check

0 steps flagged

No circularity: shear detections and redshift scaling are direct measurements compared to independent caustic masses

full rationale

The paper reports tomographic shear signals measured directly from Subaru/HSC imaging restricted to galaxies with new spectroscopic redshifts. Detections at 3.1σ and 3.5σ, amplitude consistency with caustic dynamical masses, and redshift scaling are all data-driven comparisons to external quantities. No equations, fitted parameters renamed as predictions, or self-citation chains reduce these results to the inputs by construction. The A2029 comparison is presented as an external check that flags potential systematics rather than a load-bearing premise. The derivation chain is self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review yields no explicit free parameters, axioms, or invented entities; the central claim rests on the assumption of clean spectroscopic separation and accurate shear measurement for the selected background sample.

pith-pipeline@v0.9.1-grok · 5805 in / 980 out tokens · 19032 ms · 2026-06-27T08:19:01.053061+00:00 · methodology

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