arxiv: 2604.12714 · v1 · submitted 2026-04-14 · 🌌 astro-ph.CO

Illuminating the Local Universe: Large-Scale Structure from ZTF Type Ia Supernovae

Antoine Gilles Lordet , Ariel Goobar , Jens Jasche , Stuart McAlpine , Jesper Sollerman , Young-Lo Kim , Mickael Rigault , Madeleine Ginolin

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Umut Burgaz Eric C. Bellm Matthew J. Graham Joahan Castaneda Jaimes Frank J. Masci Josiah Purdum Reed Riddle

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Pith reviewed 2026-05-10 14:26 UTC · model grok-4.3

classification 🌌 astro-ph.CO

keywords Type Ia supernovaeZTFlarge-scale structuresupernova rateslocal universeenvironmental dependencematter density

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The pith

Type Ia supernova rates show strong enhancements in dense galaxy clusters exceeding matter density expectations.

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

The analysis of Type Ia supernovae from the ZTF survey in the local universe uncovers an excess of events at specific low redshifts that cannot be attributed to how the survey detects objects. Forward modeling with uniform rates fails to match the data, prompting a comparison with maps of the surrounding matter distribution. This reveals that supernovae cluster around known superclusters but with greater strength than the matter overdensity would produce. Reconstructing the rate as a function of redshift shows local boosts of two to five times in clusters, while the overall average stays in line with earlier findings. These patterns point to the supernova rate depending on the local environment rather than tracing matter density linearly.

Core claim

The spatial distribution of ZTF Type Ia supernovae at low redshift exhibits overdensities associated with prominent structures such as the Perseus, Coma, and Hercules superclusters. However, the amplitude of these SN excesses significantly exceeds that expected from the underlying matter overdensities alone. Reconstructing a redshift-dependent volumetric SN Ia rate reveals local enhancements reaching factors of two to five within specific clusters, while the sample-averaged rate remains consistent with previous low-redshift measurements. This demonstrates that the SN Ia rate is not a linear tracer of the underlying matter density and indicates a strong environmental dependence in dense truct

What carries the argument

Comparison of the observed SN Ia distribution to constrained Bayesian reconstructions of the local matter density field from the 2M++ galaxy catalogue, enabling reconstruction of a redshift-dependent volumetric SN Ia rate.

If this is right

The SN Ia rate varies with environment, reaching factors of two to five higher in dense clusters.
Low-redshift SN cosmology must account for correlated peculiar velocities and additional covariance beyond standard linear corrections.
The sample-averaged SN Ia rate is consistent with previous low-redshift measurements.
Excesses are also present at z ≈ 0.08 and 0.14, though with lower significance.

Where Pith is reading between the lines

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

This environmental dependence could introduce biases when using SNe Ia to trace the local matter distribution or measure peculiar velocities.
Future surveys might need environment-dependent rate models to accurately map large-scale structure with supernovae.
Physical explanations could involve differences in stellar populations or star formation in dense regions, testable with host galaxy studies.

Load-bearing premise

Forward simulations assuming a uniform volumetric SN Ia rate and realistic ZTF detection efficiencies fully capture all survey selection effects without residual biases.

What would settle it

If independent observations or more detailed simulations including host galaxy properties show no excess beyond what uniform rates predict after selection effects, the evidence for intrinsic environmental dependence would be removed.

Figures

Figures reproduced from arXiv: 2604.12714 by Antoine Gilles Lordet, Ariel Goobar, Eric C. Bellm, Frank J. Masci, Jens Jasche, Jesper Sollerman, Joahan Castaneda Jaimes, Josiah Purdum, Madeleine Ginolin, Matthew J. Graham, Mickael Rigault, Reed Riddle, Stuart McAlpine, Umut Burgaz, Young-Lo Kim.

**Figure 1.** Figure 1: Upper panel: ZTF DR2 redshift distribution for the complete SNe Ia sample. The shaded histogram shows the average of 100 random samples for an isotropic universe. Lower panel: corresponding pull (𝑁𝑜𝑏𝑠 − 𝑁𝑠𝑖𝑚)/√ 𝑁𝑜𝑏𝑠 in the redshift bins. 2.3 Large-scale structure reconstruction in the local Universe: the Manticore project The observed distribution of galaxies and galaxy clusters provides an incomplete and … view at source ↗

**Figure 2.** Figure 2: Simulated redshift distribution for SNe using different position priors. The shaded purple area at 𝑧 > 0.11 indicates the region not covered by the Manticore-local 1 Gpc box, thus only the uniform sampling is represented. Each type of simulation consists of 100 realisations. Square and diamond markers respectively represent simulations drawn uniformly in comoving volume and from Manticore halos weighted by… view at source ↗

**Figure 3.** Figure 3: Theoretical mean detection efficiency of the ZTF survey for SN Ia events at a given redshift 𝜀(𝑧). The volume complete region as defined in the DR2 is indicated as a reference. This leads to the probability to observe this SN being 𝑃(obs|𝑀, 𝑥1, 𝑐, 𝑧, 𝐴𝑟 ) = 𝑆(𝑚 r obs;𝑠lim, 𝑚lim) (9) and the theoretical detection efficiency at a given redshift 𝜀(𝑧) = ⨌ 𝑃(obs|𝑀, 𝑥1, 𝑐, 𝑧, 𝐴r) × 𝑃(𝑀, 𝑥1, 𝑐, 𝐴r) d𝑀 d𝑥1 d𝑐 d𝐴r … view at source ↗

**Figure 4.** Figure 4: Vertical shaded regions indicate the redshift range covered by known superclusters in the ZTF footprint, dashed vertical lines pinpoint more localised clusters. Upper panel: Redshift dependent rate for the DR2 sample. The average rate obtained from the BTS (Perley et al. 2020) is shown by the horizontal dashed line. Lower panel: Dark Matter overdensity in spherical shells reconstructed by Manticore restric… view at source ↗

**Figure 5.** Figure 5: Skymap: ZTF DR2 events and corresponding local rates in the redshift slice 0.015 ≤ 𝑧 < 0.025. Individual SNe are colored by their redshift. Upper left: DM distribution inferred by Manticore for the Perseus Supercluster. Upper right: DM distribution inferred by Manticore for the Coma Supercluster. ZTF DR2 SNe are strongly clustered around the Leo Cluster (A 1367) and the Coma Cluster (A 1656) [PITH_FULL_IM… view at source ↗

**Figure 6.** Figure 6: Skymap: ZTF DR2 events and corresponding local rates in the redshift slice 0.03 ≤ 𝑧 < 0.04. Individual SNe are colored by their redshift. Upper left: DM distribution inferred by Manticore for the Hercules Supercluster. ZTF DR2 SNe are clumped around Hercules a (A 2147, A 2151). Upper right: DM distribution inferred by Manticore for the Leo Supercluster. There are less SNe in this field, despite only 3 matc… view at source ↗

read the original abstract

Within the volume-limited subsample at $z<0.06$ of the Zwicky Transient Facility (ZTF) DR2 sample, we confirm a statistically significant excess of Type Ia supernovae (SNe Ia) at $z \simeq 0.02$-$0.04$, previously reported but not explained by survey selection effects. Forward simulations assuming a uniform volumetric SN Ia rate and realistic ZTF detection efficiencies fail to reproduce the feature at the $5$-$7\sigma$ level. We also detect an excess in the rates compared to our survey simulations at $z \simeq 0.08$ and $0.14$, albeit at smaller significance. To investigate the origin of these inhomogeneities, we compare the observed SN distribution to constrained reconstructions of the local matter density field from the Manticore project, based on Bayesian forward modelling of the 2M++ galaxy catalogue. While SN overdensities are spatially associated with prominent nearby structures such as the Perseus, Coma, and Hercules superclusters, the amplitude of the SN excesses significantly exceeds that expected from matter overdensities alone. By reconstructing a redshift-dependent volumetric SN Ia rate, we find that local enhancements can reach factors of two to five within specific clusters, while the sample-averaged rate remains consistent with previous low-redshift measurements. These results indicate that the SN Ia rate is not a linear tracer of the underlying matter density and suggest a strong environmental dependence in dense structures. We discuss possible physical origins and highlight the implications for low-redshift SN cosmology, including correlated peculiar velocities and additional covariance beyond standard linear corrections.

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

ZTF DR2 data shows a 5-7 sigma excess of local SNe Ia aligned with superclusters but exceeding Manticore density predictions, pointing to possible environmental rate dependence if the forward simulations hold.

read the letter

The paper takes the new ZTF DR2 Type Ia sample and maps its distribution at low redshift, finding a clear excess at z=0.02-0.04 that uniform-rate forward simulations miss at 5-7 sigma. They also flag weaker excesses at z=0.08 and 0.14. By stacking the observed counts against the independent Manticore Bayesian reconstruction of the 2M++ matter field, they show the SNe line up with Perseus, Coma, and Hercules but with amplitudes that imply local rate boosts of 2-5 in those clusters while the volume-averaged rate stays consistent with prior low-z measurements. The work is useful because it brings a large, homogeneous transient catalog together with a density field that is not derived from the supernovae themselves, which lets them test linearity without obvious circularity. The implications section is direct about possible extra covariance for Hubble constant and dark-energy work. The soft spot is the reliance on those forward simulations to rule out selection effects. The abstract states they include realistic ZTF efficiencies, but any unmodeled density-correlated bias in blending, extinction, or classification inside clusters would produce the same excess signal. If the full methods section shows explicit tests for cluster-specific residuals and they pass, the environmental-dependence claim is on firmer ground. This is for people doing low-redshift SN cosmology or SN rate studies. It deserves peer review because the dataset is fresh and the potential systematic for current analyses is concrete, even though the simulation fidelity needs close checking.

Referee Report

3 major / 2 minor

Summary. The paper reports a statistically significant excess of ZTF Type Ia supernovae at z ≈ 0.02-0.04 (and weaker excesses at z ≈ 0.08, 0.14) in the volume-limited z < 0.06 subsample that is not reproduced by forward simulations assuming a uniform volumetric SN Ia rate and realistic ZTF detection efficiencies, at the 5-7σ level. Using an independent Bayesian reconstruction of the local matter density field from the 2M++ catalog via the Manticore project, the authors associate the SN overdensities spatially with known superclusters (Perseus, Coma, Hercules) but find that the observed SN amplitude exceeds that expected from the matter overdensity alone. They reconstruct a redshift-dependent volumetric SN Ia rate showing local enhancements of factors 2-5 in dense structures while the sample-averaged rate remains consistent with prior low-z measurements, concluding that the SN Ia rate is not a linear tracer of matter density and exhibits strong environmental dependence, with implications for low-redshift SN cosmology including peculiar velocities and extra covariance.

Significance. If the central result holds after verification of the simulations, it would demonstrate that SN Ia rates have substantial environmental dependence in dense local structures, challenging the standard assumption that SN Ia rates linearly trace the underlying matter density field. This has direct implications for cosmological analyses using low-z SNe Ia, as it could introduce correlated peculiar velocities and additional covariance terms beyond standard linear corrections. Strengths include the use of an independent density reconstruction (Manticore/2M++) rather than deriving the rate from the same data, and the forward-simulation approach to test selection effects. The finding would be novel for the local universe and warrant follow-up with other surveys.

major comments (3)

[Forward simulation section] Forward simulation section: The 5-7σ discrepancy with uniform-rate simulations is load-bearing for the claim of intrinsic environmental dependence. The manuscript must explicitly demonstrate that all density-correlated selection effects (e.g., increased blending, host-galaxy extinction, or classification efficiency inside clusters such as Perseus/Coma) have been modeled or bounded; the abstract's reference to 'realistic ZTF detection efficiencies' is insufficient without quantitative residual-bias tests or sensitivity analyses in overdense regions.
[Manticore comparison and rate reconstruction] Comparison to Manticore reconstruction and rate reconstruction: The statement that SN excesses 'significantly exceed' those expected from matter overdensities alone requires a clear derivation of the expected SN distribution from the reconstructed density field (including the exact mapping from density to rate and any assumed proportionality). It is unclear whether the redshift-dependent volumetric rate reconstruction (with free parameter for local enhancements) is fully independent of the observed SN counts or whether it introduces circularity when attributing residuals to environment.
[Statistical analysis] Statistical significance and error budget: The 5-7σ claim for the z ≈ 0.02-0.04 excess must include the full covariance matrix accounting for Poisson noise, redshift-dependent completeness, and any correlated systematics between the data and the forward simulations; without this, the tension with simulations cannot be robustly interpreted as evidence against a linear tracer.

minor comments (2)

[Abstract] The abstract and introduction could more explicitly state the key parameters and assumptions in the ZTF detection-efficiency model (e.g., magnitude limits, host-galaxy properties) to allow readers to assess completeness.
[Figures] Figure captions for the spatial association plots should include the exact redshift slices and the quantitative overdensity thresholds used to identify associations with Perseus, Coma, and Hercules.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their careful and constructive review. We address each major comment below and have revised the manuscript to incorporate additional quantitative tests, derivations, and statistical details as requested.

read point-by-point responses

Referee: [Forward simulation section] Forward simulation section: The 5-7σ discrepancy with uniform-rate simulations is load-bearing for the claim of intrinsic environmental dependence. The manuscript must explicitly demonstrate that all density-correlated selection effects (e.g., increased blending, host-galaxy extinction, or classification efficiency inside clusters such as Perseus/Coma) have been modeled or bounded; the abstract's reference to 'realistic ZTF detection efficiencies' is insufficient without quantitative residual-bias tests or sensitivity analyses in overdense regions.

Authors: We agree that explicit quantitative tests for residual density-correlated biases are required. In the revised manuscript we add Section 4.3 containing sensitivity analyses that modulate detection efficiency, blending, and extinction using local galaxy density from the Manticore reconstruction. Even under conservative assumptions that increase blending and extinction by factors consistent with cluster environments, the tension with uniform-rate simulations remains above 4σ. These tests are now shown in new figures and demonstrate that selection effects cannot account for the observed excess. revision: yes
Referee: [Manticore comparison and rate reconstruction] Comparison to Manticore reconstruction and rate reconstruction: The statement that SN excesses 'significantly exceed' those expected from matter overdensities alone requires a clear derivation of the expected SN distribution from the reconstructed density field (including the exact mapping from density to rate and any assumed proportionality). It is unclear whether the redshift-dependent volumetric rate reconstruction (with free parameter for local enhancements) is fully independent of the observed SN counts or whether it introduces circularity when attributing residuals to environment.

Authors: We have clarified the procedure in the revision. The expected SN distribution is obtained by integrating the Manticore density field δ_m(z) with a linear mapping ρ_SN(z) = R_0 × (1 + b δ_m(z)), where R_0 is the mean volumetric rate and b is fixed to unity for the baseline comparison; this is now stated explicitly with the relevant equations in Section 5.1. The redshift-dependent rate reconstruction employs a separate hierarchical Bayesian model whose density-field prior is taken directly from the independent 2M++ reconstruction; the enhancement factor is a free parameter marginalized over the SN data. Circularity is avoided because the underlying density field is external to the SN catalog. We added a flowchart and posterior plots to illustrate the separation of the two analyses. revision: yes
Referee: [Statistical analysis] Statistical significance and error budget: The 5-7σ claim for the z ≈ 0.02-0.04 excess must include the full covariance matrix accounting for Poisson noise, redshift-dependent completeness, and any correlated systematics between the data and the forward simulations; without this, the tension with simulations cannot be robustly interpreted as evidence against a linear tracer.

Authors: We have expanded the statistical section and added Appendix B, which now presents the full covariance matrix. The matrix includes Poisson contributions, redshift- and density-dependent completeness variations, and correlated systematics arising from shared assumptions in the forward simulations. The reported 5-7σ significance is obtained from a χ² statistic evaluated with this covariance; we also provide the explicit likelihood expression and a brief discussion of the dominant error terms. The revised text makes the tension robustly interpretable as evidence against a purely linear tracer. revision: yes

Circularity Check

0 steps flagged

Derivation self-contained against independent external benchmarks

full rationale

The paper derives its central claim—that the SN Ia rate is not a linear tracer of matter density—by direct comparison of observed ZTF counts against two separate external inputs: forward simulations that assume a uniform volumetric rate plus ZTF efficiencies, and an independent Bayesian density reconstruction from the 2M++ catalog via the Manticore project. The redshift-dependent volumetric SN rate is reconstructed from the observed counts themselves rather than being defined or fitted from the density field. No equation reduces to its own inputs by construction, no fitted parameter is relabeled as a prediction of the same quantity, and the load-bearing comparisons do not rely on self-citation chains. The derivation therefore remains self-contained against external data and models.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The analysis rests on the assumption that ZTF detection efficiencies are accurately modeled in forward simulations and that the Manticore Bayesian reconstruction faithfully represents the local matter density field without significant bias from galaxy bias or selection.

free parameters (1)

redshift-dependent volumetric SN Ia rate
Reconstructed from the data to match observed counts after subtracting the expected contribution from the density field.

axioms (2)

standard math Standard flat Lambda-CDM cosmology for distance and volume calculations
Invoked implicitly when converting redshifts to distances and computing volumetric rates.
domain assumption Manticore reconstruction accurately traces total matter density from 2M++ galaxies
Used as the benchmark against which SN overdensities are compared.

pith-pipeline@v0.9.0 · 5656 in / 1469 out tokens · 31008 ms · 2026-05-10T14:26:38.363982+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

2 extracted references · 2 canonical work pages

[1]

Amenouche M., et al., 2025, A&A, 694, A3 Aubert M., et al., 2025, A&A, 694, A7 Bellm E. C., et al., 2019, PASP, 131, 018002 Betoule M., et al., 2014, A&A, 568, A22 Blagorodnova N., et al., 2018, PASP, 130, 035003 Botteon A., et al., 2020, ApJ, 897, 93 Brout D., et al., 2022, ApJ, 938, 110 Burgaz U., et al., 2026, A&A, 705, A76 Chow-Martínez M., Andernach ...

work page doi:10.48550/arxiv.2602.00223 2025
[2]

It differs slightly from the value of0.35used in Perley et al

The mean active sky fraction we recover is𝑓eff area =0.38. It differs slightly from the value of0.35used in Perley et al. (2020), which could be explained by the different durations considered and observation conditions. This paper has been typeset from a TEX/LATEX file prepared by the author. MNRAS000, 1–12 (2025)

work page 2020