---
title: 'HSC-Y3: Subaru Cosmology Survey'
url: https://www.emergentmind.com/topics/hyper-suprime-cam-hsc-y3
type: topic
---

# HSC-Y3: Subaru Cosmology Survey

The Hyper Suprime-Cam Year 3 (HSC-Y3) program refers to the third annual cosmological analysis of wide-field imaging data from the Subaru Telescope’s Hyper Suprime-Cam (HSC). HSC-Y3 delivers high-precision weak lensing and clustering measurements across 416 deg² to an i-band depth of 24.5 mag, exploiting a source density of up to 20 arcmin⁻² in four tomographic redshift bins. The survey underlies a suite of joint and single-probe cosmological constraints, including cosmic shear two-point correlation functions (2PCFs), power spectrum analyses, 3×2pt methods combining clustering and lensing, and—for the first time—incorporates third-order statistics such as aperture-mass skewness. HSC-Y3 is notable for its control of systematic errors and rigorous blinding procedures, setting a benchmark for contemporary ground-based cosmology.

## 1. Survey Design and Data Products

HSC-Y3 uses data from the Subaru Strategic Program S19A release (2014–2019), covering six disjoint fields after masking regions affected by PSF or B-mode systematics [2304.00703]. Imaging is conducted across five bands (g, r, i, z, y), with the i-band seeing at ∼0.6″ and 5σ point-source depth approaching i≃26 mag [2304.00703]. 

The main HSC-Y3 shape catalog contains approximately 25 million galaxies with reliable shape measurements, yielding an effective number density of n_eff ≈ 15–20 arcmin⁻² depending on redshift bin [2304.00702, 2403.20323]. Shape measurements utilize re-Gaussianization corrections and are calibrated via extensive image simulations, achieving multiplicative shear bias |m|≲0.01 [2304.00703, 2508.14019].

Photometric redshifts are inferred via three independent codes (DNNz, DEmPZ, Mizuki), each producing per-galaxy posteriors [2211.16516]. Calibration is accomplished using cross-correlations with CAMIRA luminous red galaxies (LRGs) and hierarchical Bayesian inference, establishing Gaussian priors on the mean redshift shift (Δz) for bins 1-2 and broad flat priors for bins 3-4 [2211.16516]. The tomographic bin edges are 0.3<z≤0.6, 0.6<z≤0.9, 0.9<z≤1.2, 1.2<z≤1.5 [2403.20323, 2304.00701].

## 2. Cosmological Measurement Methodologies

HSC-Y3 supports two principal frameworks for cosmic shear analysis: real-space two-point correlation functions (2PCFs) ξ_±(θ) [2304.00702, 2403.20323] and pseudo-C_ℓ power spectra [2304.00701]. Measurements are performed on angular scales from θ=0.28′ up to 333′ in 24 log-spaced bins [2403.20323, 2508.14019]. Shear components (γ_t, γ_×) are weighted and corrected for PSF effects, with systematic error mitigation extending to B-mode decompositions and star–galaxy cross-correlation null tests [2304.00701, 2508.14019].

For joint analyses, HSC-Y3 provides 3×2pt frameworks combining projected clustering w_p(R), galaxy–galaxy lensing ΔΣ(R), and cosmic shear ξ_±(θ) [2304.00704, 2304.00705]. Theoretical modeling employs the Dark Emulator for the halo model down to quasi-nonlinear scales [2304.00704], and a minimal-bias model restricted to linear scales for w_p and ΔΣ [2304.00705]. Assembly bias, baryonic effects, and intrinsic galaxy alignments are modeled and marginalized across analysis pipelines [2304.00704, 2304.00701].

Mock catalogs are central for covariance estimation, exploiting 1404 full-sky ray-tracing N-body simulations adapted to the HSC-Y3 footprint and source-lens geometry [2304.00702, 2304.00701, 2508.14019].

## 3. Photometric Redshift Calibration and Error Marginalization

Redshift uncertainty constitutes a primary systematic in weak lensing analyses. HSC-Y3’s redshift inference leverages a Bayesian hierarchical mixture of photometric posteriors and clustering-redshift cross-correlations [2211.16516]. Ensemble redshift distributions n_i(z) for each tomographic bin are robustly inferred and prior-constrained:

| Bin | Mean z (Joint Inference) | Prior Width (σ_Δz) |
|-----|-------------------------|--------------------|
| 1   | 0.452 ± 0.004           | ±0.024             |
| 2   | 0.766 ± 0.003           | ±0.022             |
| 3   | 1.081 ± 0.004           | ±0.031             |
| 4   | ... (no WX)             | ±0.034             |

These Gaussian priors are implemented as n_i(z) → n_i(z−Δz_i) in all cosmology likelihoods, propagating photo-z uncertainty into joint inference [2211.16516].

Small-scale galaxy–galaxy lensing shear ratios provide an independent geometric constraint on photo-z bias shifts, particularly for bins 3 and 4 where traditional calibration is weakest [2508.21681]. The blinded shear-ratio results yield Δz_3=–0.002^{+0.085}_{–0.217}, Δz_4=–0.292^{+0.229}_{–0.324}, in good agreement with standard cosmic shear calibrations [2508.21681].

## 4. Cosmological Results and S₈ Tension

The leading HSC-Y3 weak lensing analyses converge on percent-level constraints for the amplitude parameter S₈≡σ₈(Ω_m/0.3)^0.5, with substantial internal and external consistency:

- 2PCF analysis: S₈=0.769^{+0.031}_{–0.034}, Ω_m=0.256^{+0.056}_{–0.044} [2304.00702]
- Pseudo-C_ℓ: S₈=0.776^{+0.032}_{–0.033}, Ω_m=0.219^{+0.075}_{–0.052} [2304.00701]
- 3×2pt halo-model: S₈=0.763^{+0.040}_{–0.036}, Ω_m=0.382^{+0.031}_{–0.047} [2304.00704]
- Minimal-bias large-scale: S₈=0.775^{+0.043}_{–0.038} [2304.00705]
- Cosmic shear + shear ratios: S₈=0.760^{+0.044}_{–0.145}, Ω_m=0.286_{–0.074}^{+0.038} [2508.21681]
- Joint 2+3pt (skewness): S₈=0.736±0.020, Ω_m=0.277±0.040 [2508.14019]

These constraints are internally robust to a wide range of scale cuts, IA models, sampler choices, and baryonic feedback prescriptions (systematic shifts in S₈ <0.5σ) [2304.00701, 2403.20323]. Third-order shear statistics (aperture-mass skewness) improve the Ω_m–S₈ figure of merit by ∼80%, enhancing degeneracy breaking and increasing tension with Planck to >3σ [2508.14019].

No significant baryonic suppression or feedback signature is observed down to θ_min=0.28′ (k≈20 h Mpc⁻¹), and the measured ∼5% suppression in P_m(k=1 h Mpc⁻¹) is insufficient to reconcile the S₈ tension with Planck, which would require ∼25% suppression at these scales [2403.20323]. All analyses consistently report a 2–3σ downward tension in S₈ relative to the Planck CMB result (S₈≈0.83), reinforcing the mild “low-z/Planck tension” [2304.00701, 2508.14019].

## 5. Systematic Controls and Validation Strategies

HSC-Y3 achieves its systematics robustness via multi-tiered validation:

- **Blinding**: Analysis-level blinding of shear catalogues and cosmology chains to avoid confirmation bias [2304.00701, 2304.00702].
- **PSF Modeling**: Residual leakage and modeling errors are parameterized and marginalized, with dedicated star–galaxy null tests, achieving additive bias well below statistical errors [2304.00701, 2508.14019].
- **Shear Calibration**: Multiplicative bias controlled and validated via image simulations; parameter priors Δm∼N(0,0.01) [2304.00703, 2304.00701].
- **Photo-z Validation**: Hierarchical Bayesian inference, clustering-z cross-correlation, and shear-ratio calibration [2211.16516, 2508.21681].
- **Intrinsic Alignments**: Marginalized via nonlinear models (NLA, TATT), with flat broad priors; found to shift S₈ <0.25σ [2304.00701].
- **Covariance**: Calculation from thousands of full-fidelity mock catalogs, including shape noise, sample variance, and super-sample covariance [2304.00702, 2304.00701].
- **Null Tests**: Internal splits (field, bin, scale), B-mode decomposition, jackknife removals all yield subdominant shifts in S₈ and Ω_m [2304.00701, 2508.14019].

## 6. Methodological Innovations and Impact

Several methodological milestones distinguish HSC-Y3:

- Implementation of compressed third-order shear statistics (M_ap³) for cosmology, demonstrating 80% improvement in joint Ω_m–S₈ constraints [2508.14019].
- Use of small-scale shear-ratio tests as a bias-free geometric probe for photo-z calibration [2508.21681].
- Emulator-based halo modeling and minimal-bias linear modeling provide cross-validation between small- and large-scale analyses [2304.00704, 2304.00705].
- Extensive pipeline-level blinding and systematics marginalization establish a best-practices template for Stage-IV experiments (LSST, Euclid, Roman) [2304.00701, 2508.14019].

## 7. Implications and Future Prospects

HSC-Y3 delivers high-precision, systematics-controlled constraints on Ω_m and S₈, verifying the mild tension with Planck CMB cosmology and demonstrating that baryonic feedback is unlikely to be the sole cause [2403.20323]. Third-order and shear-ratio–calibrated statistics represent maturing tools for future surveys. For HSC Y5, the anticipated expansion to ∼1 100 deg² and improved redshift calibration are projected to reduce S₈ uncertainty to <0.025, substantially improving cosmological leverage [2304.00702]. Integration with spectroscopic samples (DESI, PFS) and multi-band imaging will further strengthen redshift systematics control, bolstering cosmological inference in the forthcoming Stage IV era.

Source: https://www.emergentmind.com/topics/hyper-suprime-cam-hsc-y3