4MOST Cosmology Redshift Survey (CRS): Clustering Properties of CRS Bright Galaxy and Luminous Red Galaxy Target Catalogues

Antoine Rocher; Aur\'elien Verdier; Behnood Bandi; Jean-Paul Kneib; Johan Richard; Jon Loveday; Michael J. I. Brown; Tom Shanks; Zhuo Chen

arxiv: 2510.02449 · v4 · submitted 2025-10-02 · 🌌 astro-ph.CO · astro-ph.GA

4MOST Cosmology Redshift Survey (CRS): Clustering Properties of CRS Bright Galaxy and Luminous Red Galaxy Target Catalogues

Behnood Bandi , Antoine Rocher , Aur\'elien Verdier , Jon Loveday , Zhuo Chen , Johan Richard , Jean-Paul Kneib , Tom Shanks

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Michael J. I. Brown

This is my paper

Pith reviewed 2026-05-18 10:09 UTC · model grok-4.3

classification 🌌 astro-ph.CO astro-ph.GA

keywords 4MOSTCosmology Redshift Surveylarge-scale structuretarget selectionangular correlation functionbright galaxiesluminous red galaxiesmasking

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The pith

Masks for 4MOST bright galaxy and luminous red galaxy targets suppress small-scale excess power without introducing large-scale biases.

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

The paper tests whether the selected bright galaxy and luminous red galaxy samples from the DESI Legacy Survey can serve as clean targets for the 4MOST Cosmology Redshift Survey. It measures angular two-point correlation functions, applies and compares masking choices, and recovers redshift distributions through cross-correlation with existing spectroscopy. These checks matter because the survey plans to use millions of redshifts to measure baryon acoustic oscillations and redshift-space distortions, which require target catalogues free of selection-induced artifacts on both small and large scales. The results show that the adopted masks achieve this balance while maintaining uniformity across magnitude slices and survey footprints.

Core claim

Adoption of LS MASKBITS for bright galaxies combined with unWISE W1 artefact masks for luminous red galaxies removes small-scale excess power in the angular correlation function without imprinting spurious large-scale modes. After Limber scaling, correlation curves from different r-band magnitude slices collapse onto a common power law, and cross-correlations with DESI DR1 recover the expected redshift distributions. Angular clustering in photo-z slices for LRGs remains consistent between DECaLS and DES footprints and is well fit by halo occupation distributions that match other recent LRG samples.

What carries the argument

Angular two-point correlation function w(θ) together with masking tests and cross-correlation redshift recovery, used to verify photometric uniformity and absence of systematics in the target catalogues.

If this is right

The validated BG and LRG catalogues support measurements of baryon acoustic oscillations and growth rates via redshift-space distortions.
Recovered redshift distributions match expectations and can be used for survey planning.
Clustering in photo-z slices for LRGs is consistent across different imaging footprints once photo-z smearing is accounted for.
Halo occupation distribution fits align with those reported in other recent LRG studies.

Where Pith is reading between the lines

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

The same masking approach could be adapted for target selection in future wide-field spectroscopic surveys that draw from similar imaging.
Clean angular clustering opens the possibility of joint analyses with weak-lensing maps to tighten constraints on structure growth.
Extension to higher-order correlation functions would provide an additional test of the absence of residual systematics.

Load-bearing premise

The chosen masking strategies suppress small-scale excess power without introducing biases or large-scale modes that would affect cosmological measurements.

What would settle it

Observation of inconsistent large-scale power in w(θ) or mismatched clustering amplitude between the North and South Galactic Caps after applying the masks would indicate that the target selections are not reliable for precision analyses.

read the original abstract

The 4-metre Multi-Object Spectroscopic Telescope Cosmology Redshift Survey (4MOST CRS) will obtain 5.4 million spectroscopic redshifts over ~5700 deg^2 to map large-scale structure and enable measurements of baryon acoustic oscillations, growth rates via redshift-space distortions, and cross-correlations with weak-lensing surveys. We validate the target selections, photometry, masking, systematics, and redshift distributions of the bright galaxy (BG) and luminous red galaxy (LRG) target catalogues selected from the Dark Energy Spectroscopic Instrument (DESI) Legacy Survey DR10.1 (LS) imaging. We measure the angular two-point correlation function, $w(\theta)$, test masking strategies, and recover redshift distributions via cross-correlation with DESI DR1 spectroscopy. For BGs, we adopt LS MASKBITS that veto bright stars and extended sources; for LRGs, we pair these with unblurred coadds of the WISE imaging (unWISE) W1 artefact masks. These choices suppress small-scale excess power without imprinting large-scale modes. A Limber-scaling test across BG $r$-band magnitude slices shows that, after applying the scaling, the $w(\theta)$ curves collapse to a near-common power law, demonstrating photometric uniformity with depth and consistency between the North and South Galactic Caps. Cross-correlations with DESI spectroscopy recover the expected N(z), albeit with high shot noise at the brightest magnitudes. For LRGs, angular clustering in photo-$z$ slices ($0.4\le z<1.0$) is consistent between the Dark Energy Camera Legacy Survey (DECaLS) and Dark Energy Survey (DES) footprints and is well described by an approximate power law once photo-$z$ smearing is included; halo-occupation fits are consistent with recent LRG studies. Together, these tests indicate that the masks and target selections yield reliable clustering statistics, supporting precision large-scale structure analyses with 4MOST CRS.

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.

Desk Editor's Note private letter to a colleague

This paper runs standard clustering checks on the 4MOST bright galaxy and LRG target catalogs and finds the masks and selections produce consistent results with no obvious large-scale problems.

read the letter

The main takeaway is that the 4MOST CRS target selections for bright galaxies and luminous red galaxies hold up under basic angular clustering tests using DESI Legacy Survey data. The authors apply LS MASKBITS to veto stars and extended objects for the BGs and add unWISE W1 artefact masks for the LRGs. These choices remove small-scale excess power while leaving large-scale amplitudes consistent across the NGC and SGC footprints. A Limber scaling test across r-band magnitude slices collapses the w(theta) curves to a common power law, which supports photometric uniformity with depth. Cross-correlations with DESI DR1 spectroscopy recover the expected N(z) distributions, and for the LRGs the photo-z slice clustering plus HOD fits match patterns seen in earlier work. This is useful survey-preparation material because it documents that the catalogs should support clean BAO and RSD measurements once 4MOST starts observing. The tests rely on public external datasets and established techniques rather than new methods, so the advance is incremental but directly relevant to the upcoming survey. The soft spots are limited. The analysis is mostly consistency checks against prior expectations, and the abstract leaves some details on error estimation and full systematic budgets for the full text. The assumption that the chosen masks avoid subtle cosmological biases is the weakest link, though the large-scale consistency across footprints helps mitigate it. No load-bearing contradictions appear in the described chain. This paper is mainly for the 4MOST team and others doing target selection or systematics work for similar wide-field spectroscopic surveys. A reader preparing for precision large-scale structure analyses would find the masking and uniformity tests practical to reference. It deserves a serious referee to check the full figures, covariance handling, and any residual photo-z effects before the survey data start flowing.

Referee Report

0 major / 3 minor

Summary. The manuscript validates the bright galaxy (BG) and luminous red galaxy (LRG) target catalogues for the 4MOST Cosmology Redshift Survey selected from DESI Legacy Survey DR10.1 imaging. It measures the angular two-point correlation function w(θ), tests masking strategies (LS MASKBITS for BGs paired with unWISE W1 artefact masks for LRGs), recovers redshift distributions N(z) via cross-correlation with DESI DR1 spectroscopy, performs Limber scaling across r-band magnitude slices to demonstrate photometric uniformity and NGC/SGC consistency, and fits halo occupation distributions to LRG clustering in photo-z slices (0.4 ≤ z < 1.0). The central claim is that these masks and selections suppress small-scale contaminants without introducing large-scale biases, yielding reliable clustering statistics that support precision large-scale structure analyses with 4MOST CRS.

Significance. If the validations hold, this work is significant for enabling the 4MOST CRS science goals of mapping large-scale structure over ~5700 deg² with 5.4 million redshifts to measure BAO, growth via RSD, and cross-correlations with weak lensing. The paper provides concrete evidence from public datasets (DESI Legacy Survey, DESI DR1) that the target selections are robust, with strengths including the use of external spectroscopy for N(z) recovery, footprint consistency checks (DECaLS vs. DES, NGC vs. SGC), and HOD fits matching prior LRG studies. These standard but necessary tests reduce risk for the survey's cosmological analyses.

minor comments (3)

The description of masking for LRGs references 'unblurred coadds of the WISE imaging (unWISE) W1 artefact masks'; a short explanation of the unblurred coadd processing and why it is paired with LS MASKBITS would improve clarity for readers.
In the Limber-scaling test across BG r-band slices, the manuscript states that the w(θ) curves 'collapse to a near-common power law' after scaling; specifying the exact scaling applied (e.g., the multiplicative factor or reference depth) and the angular scale range used for the comparison would make the photometric uniformity claim more quantitative.
The cross-correlation recovery of N(z) notes high shot noise at the brightest magnitudes; adding a brief assessment of how this noise level propagates into the validation of the brightest BG subsample would strengthen the overall reliability argument.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive and accurate summary of the manuscript, which correctly identifies the key validations of the BG and LRG target catalogues, including angular clustering, masking tests, N(z) recovery via cross-correlation, Limber scaling, and HOD fits. We appreciate the acknowledgment of the work's significance for enabling BAO, RSD, and weak-lensing cross-correlation measurements with 4MOST CRS. The recommendation for minor revision is noted, and we will implement editorial improvements and clarifications in the revised version.

Circularity Check

0 steps flagged

No significant circularity

full rationale

The manuscript performs empirical validation of target selections and masks using direct measurements of angular clustering w(θ), cross-correlations with independent DESI DR1 spectroscopy to recover N(z), Limber scaling tests for photometric uniformity, and consistency checks across DECaLS/DES footprints and NGC/SGC. All load-bearing results are obtained from public external datasets rather than internal fits, self-referential definitions, or author-cited uniqueness theorems. No step reduces a claimed prediction or central result to its own inputs by construction, so the reliability conclusions remain self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

This is an observational validation study using standard techniques on public survey data. No new free parameters, ad-hoc axioms, or invented entities are introduced beyond routine assumptions in angular clustering analysis.

axioms (2)

standard math Limber approximation holds for projecting clustering to angular scales in the tested magnitude and redshift ranges
Invoked in the Limber-scaling test across BG r-band slices to demonstrate uniformity.
domain assumption Cross-correlation with DESI DR1 spectroscopy recovers unbiased N(z) for the target samples
Used to validate redshift distributions despite high shot noise at bright magnitudes.

pith-pipeline@v0.9.0 · 5945 in / 1406 out tokens · 50546 ms · 2026-05-18T10:09:14.428761+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear

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unclear
Relation between the paper passage and the cited Recognition theorem.

We measure the angular two-point correlation function, w(θ), test masking strategies, and recover redshift distributions via cross-correlation with DESI DR1 spectroscopy.
IndisputableMonolith/Foundation/DimensionForcing.lean alexander_duality_circle_linking unclear

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unclear
Relation between the paper passage and the cited Recognition theorem.

A Limber-scaling test across BG r-band magnitude slices shows that, after applying the scaling, the w(θ) curves collapse to a near-common power law

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