REVIEW 3 major objections 3 minor 48 references
Evidence for Intrinsic Galaxy Alignments in Ellipticity Autocorrelations out to $100 h^{-1}\textrm{Mpc}$ from SDSS Galaxies with DESI Imaging
T0 review · 3 major / 3 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read This paper reports the first observational evidence that the autocorrelation of galaxy ellipticities—the intrinsic–intrinsic correlation—follows a clean power law out to 100 Mpc in three SDSS galaxy samples.
desk verdict A plausible first 100 Mpc II autocorrelation measurement with a clever basis choice, but the missing PSF/systematics budget keeps the headline claim from being fully secured. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is the $L=4$ multipole of the II(−) correlation, $\tilde{\xi}_{-,L}(r)$, obtained by expanding the density-weighted ellipticity autocorrelation in normalized associated Legendre polynomials $\Theta_L^{m=4}(\mu_r)$. Because the line-of-sight projection factor $(1-\mu_k^2)^2$ in the NLA power spectrum appears only in the II(−) statistic, the linear-order signal compresses into a single multipole, unlike the standard Legendre expansion that spreads it over $\ell=0,2,4$. On the model side, the paper uses the nonlinear alignment model with a shape-bias parameter $b_K$ and a finger-of-God damping term, with the nonlinear matter power spectrum supplied by revised Halofit.
What would settle it
Rotate every galaxy ellipticity by 45 degrees to form a B-mode estimator, or replace the galaxy shapes with the shapes of unresolved stars at the same positions, and measure the same $L=4$ correlation out to $100\,h^{-1}\mathrm{Mpc}$; if a comparable power-law signal appears, the claimed detection is dominated by additive systematics rather than intrinsic alignments.
Extended reading notes
Core claim
The central claim is that the II(−) ellipticity autocorrelation, expanded in the associated Legendre basis, is detected as a smooth power law out to $100\,h^{-1}\mathrm{Mpc}$ in all three samples. Fitting the nonlinear alignment (NLA) model with a Gaussian finger-of-God damping to the $L=4$ multipole $\tilde{\xi}_{-,L}(r)$ gives shape-bias amplitudes $-b_K\sigma_8 = 0.2358$, $0.1198$, and $0.1174$ for LRG, LOWZ, and CMASS, respectively, with quoted errors near $0.01$. The paper also shows that combining II(+) and II(−) improves the detection significance by about 10% even though both are derived from the same $E$-mode power spectrum. These results are presented as the first unambiguous measurement of intrinsic shape autocorrelations at BAO-like scales, achieved by the basis choice and by the deeper DESI imaging.
Load-bearing premise
The measurement is read as intrinsic galaxy alignments, so the ellipticity autocorrelation must be dominated by true galaxy shapes rather than by observational systematics such as image-blurring distortions or weak-lensing shear; the analysis does not report a point-spread-function correction or null-test step to exclude those contaminants.
Editorial extensions
If this is right
- Galaxy shape autocorrelations can now be used as a large-scale structure probe at BAO-like scales, not just as a small-scale signal.
- Combining the II(+) and II(−) statistics yields about a 10% gain in detection significance, even though both derive from the same $E$-mode power spectrum.
- The associated-Legendre basis puts the entire linear-order II(−) signal into a single multipole, so it is the natural expansion for future intrinsic-alignment analyses.
- Shape autocorrelations open a new observational window on signals uniquely encoded in them, such as tensor perturbations from a stochastic gravitational-wave background.
- The same framework can be extended to wider redshift ranges and larger volumes with newer imaging data.
Reading between the lines
- If the detection survives a B-mode null test and point-spread-function contamination checks—neither is reported here—the quoted amplitudes are intrinsic; otherwise they are upper limits on the combined intrinsic-alignment and systematic signal.
- At 100 Mpc scales, intrinsic alignments are large enough that cosmic-shear surveys will need to model II correlations out to BAO-like scales, not only on small scales.
- The associated-Legendre compression may transfer to other projected spin-2 statistics, such as cosmic shear or CMB polarization, where a single multipole could capture the leading signal.
- If the NLA model truly fits the full range, the scale dependence of $b_K$ can now be measured out to 100 Mpc, testing tidal-alignment theory in a previously inaccessible regime.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper measures the intrinsic-intrinsic (II) ellipticity autocorrelation function from three SDSS galaxy samples (LRG, LOWZ, CMASS) over 0.16≤z≤0.70, using DESI Legacy Survey imaging for the LOWZ and CMASS shapes. The authors expand the II(−) correlation in the associated Legendre basis and report that the L=4 mode shows a clear power-law trend out to 100 h−1 Mpc for all three samples. They fit a nonlinear alignment (NLA) model with parameters (bKσ8, σv) and obtain reduced χ2 values near unity for the II(−) associated-Legendre fits. They claim this is the first observational evidence for intrinsic alignments in ellipticity autocorrelations at these scales.
Significance. The claimed result, if robust, would be a notable observational advance: it would extend shape autocorrelation measurements to BAO-like scales and open a new probe of signals such as gravitational-wave imprints. The associated-Legendre expansion is well motivated and clearly presented, and the consistency of a power-law signal across three independent samples is encouraging. However, the detection claim is not yet secured because the paper reports no systematics tests (PSF null tests, B-mode tests, or cosmic-shear subtraction) and gives no quantitative detection significance. The strengths are the novel basis choice and the use of deeper imaging for LOWZ and CMASS; the weaknesses are the missing systematics budget and the absence of a model-independent significance statement.
major comments (3)
- [Sections 2 and 3, Eqs. (1)-(3)] The estimator in Eq. (3) treats the measured shapes as intrinsic alignments, but no PSF correction, stellar-shape null test, B-mode test, or cosmic-shear subtraction is reported for any of the three samples. Because the linear-order II(−) signal is compressed into a single associated-Legendre mode, a coherent PSF anisotropy correlated on degree scales could produce a spurious power-law signal in eξ−,L. Please add null tests (e.g., cross-correlation of galaxy shapes with stellar shapes, a B-mode measurement, and a test against PSF size or seeing) or explicitly quantify the maximum allowable systematics amplitude.
- [Section 4.2, Table 1, and Abstract] No detection significance is stated anywhere in the paper. The abstract claims that joint analysis increases the detection significance by ~10%, but the significance is not defined; the quoted bKσ8 errors are model-dependent and do not by themselves quantify the evidence against the null hypothesis bK=0. Please report for each sample the Δχ2 or signal-to-noise ratio of the eξ−,L measurement against the null, and use that quantity to support the 'first evidence' claim.
- [Section 4.2, CMASS II(+) fit] The II(+) standard-Legendre fit for CMASS has reduced χ2=2.909 (Table 1), i.e., a poor fit. The text states that excluding data with rmin=25 h−1 Mpc brings χ2/dof to ~1, but this alternative fit is not shown in Table 1, and the joint analyses (rows 4 and 5 of Table 1) do not specify which scale cuts are used for each statistic. Please make the scale cuts explicit for every entry in Table 1 and report the resulting χ2 and parameter constraints for the rmin=25 case.
minor comments (3)
- [Sections 1 and 2] The instrument is referred to as the 'Dark Energy Spectrograph Instrument' twice; the correct name is the Dark Energy Spectroscopic Instrument (DESI).
- [Section 4.1 and Figures 1-2] The 'Linear' predictions use the same nonlinear Halofit power spectrum as the 'Nonlinear' model and differ only in the FoG damping; please clarify this in the text or label the curves as 'Linear RSD' and 'Nonlinear RSD' to avoid confusion.
- [Section 3] Please provide a brief statement on the jackknife implementation, e.g., the number of jackknife regions and the geometry, so that the covariance estimate is reproducible.
Circularity Check
No significant circularity: the II measurements are direct data products, and the NLA model is fitted to them rather than used to generate the claimed signal.
full rationale
The core detection is not circular. The ellipticity autocorrelation functions ξ±(r) are measured directly from galaxy shapes via the pair-based estimator in Eq. (3); they do not depend on the NLA model or on any fitted parameter. The NLA model appears only in Sec. 4.1, where it is used to interpret the measured multipoles, and in Sec. 4.2 the free parameters (bKσ8, σv) are fitted to the data (Table 1). Thus the quoted amplitudes are parameter estimates, not predictions derived from the same quantities in a way that would force the conclusion. The associated-Legendre expansion of Eq. (5) is a linear recombination of the measured ξ−(r, μr), and the claim that the linear-order signal is concentrated in L=4 follows from the E-mode power-spectrum expression in Eq. (8) and the Appendix formulas (Eqs. A1–A4), which are given in the paper rather than imported solely from a self-citation. The use of previous work, including Okumura et al. (2024) and Okumura & Taruya (2023), is not load-bearing in a circular sense: the model formulas are reproduced in the Appendix, and the earlier LRG measurement is an independent earlier data product. The statement that the improved detection is 'entirely due to the improved basis choice' is an assessment of the analysis method, not a claim that the data were generated by the model. Concerns about PSF anisotropy or lensing contamination would be systematics issues, not circularity, because the signal is not defined in terms of, or constructed from, the model being tested. No step in the derivation reduces by definition to its own input.
Assumptions & free parameters
free parameters (3)
- bK sigma8 (shape bias amplitude times sigma8) =
LRG II(-) associated: -bK sigma8 = 0.2358 +/- 0.0170; LOWZ: 0.1198 +/- 0.0102; CMASS: 0.1174 +/- 0.0083
- sigma_v (nonlinear velocity dispersion) =
Not quoted as a point estimate; contours shown for LOWZ in Figure 3
- r_min scale cut for CMASS II(+) fit =
25 h^-1 Mpc (default is 6 h^-1 Mpc)
assumptions (5)
- domain assumption NLA model: intrinsic ellipticity is linearly proportional to the nonlinear tidal field (Equation 6, after Catelan et al. 2001; Hirata and Seljak 2004).
- domain assumption Jackknife resampling yields reliable covariance when shot noise dominates.
- domain assumption SDSS and DESI imaging shape measurements accurately represent intrinsic galaxy orientations, with no significant PSF anisotropy, additive bias, or cosmic shear contamination.
- domain assumption Flat LCDM cosmology from Planck 2018 is adopted.
- standard math Line-of-sight projection of a spin-2 field leads to an associated Legendre expansion with m=4.
Cite this review
Pith. "Pith review of Evidence for Intrinsic Galaxy Alignments in Ellipticity Autocorrelations out to $100 h^{-1}\textrm{Mpc}$ from SDSS Galaxies with DESI Imaging." pith.science (2026). https://pith.science/paper/IA2NBAX7
@misc{pith2026250709756,
author = {Pith},
title = {Pith review of: Evidence for Intrinsic Galaxy Alignments in Ellipticity Autocorrelations out to $100 h^-1\textrmMpc$ from SDSS Galaxies with DESI Imaging},
year = {2026},
howpublished = {\url{https://pith.science/paper/IA2NBAX7}},
note = {Machine review of arXiv:2507.09756}
}
abstract
Measuring the autocorrelation of galaxy shapes, known as the intrinsic-intrinsic (II) correlation, is important for both cosmology and understanding the formation of massive elliptical galaxies. However, such measurements are significantly more challenging than those of the cross-correlation with galaxy density (GI correlation) due to the much lower signal-to-noise ratio. In this Letter, we report the first observational evidence for large-scale intrinsic alignments measured from the ellipticity autocorrelations, extending out to $100\,h^{-1}\,{\rm Mpc}$. From the Sloan Digital Sky Survey (SDSS) and SDSS-III Baryon Oscillation Spectroscopic Survey, we analyze, over the redshift range $0.16\leq z\leq 0.70$, luminous red galaxy, LOWZ, and CMASS galaxy samples, the latter two of which are crossmatched with high-quality Dark Energy Spectrograph Instrument imaging data. By expanding one of the two II correlation functions, II($-$), in terms of the associated Legendre polynomials, we effectively isolate the line-of-sight projection effects and enhance the signal. The resulting correlation for all three samples exhibits a clear power-law form. We also show that jointly analyzing the two II correlations, II($+$) and II($-$), increases the detection significance by $\sim 10\%$, even though both are derived from the same $E$-mode power spectrum. Importantly, this measurement opens a new observational window for probing signals uniquely encoded in shape autocorrelations, such as tensor perturbations from the gravitational waves. Our analysis establishes a practical framework for extracting such effects.
Figures
Reference graph
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Reviewed August 6, 2026 · model on record in the stance chip above.
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