[Paper Review] Hyper Suprime-Cam Year 3 Results: Cosmology from Cosmic Shear Power Spectra

Roohi Dalal, Xiangchong Li+36 more|arXiv (Cornell University)|Apr 3, 2023

Galaxies: Formation, Evolution, Phenomena179 references25 citations

TL;DR

The paper measures weak lensing cosmic shear power spectra from the Hyper Suprime-Cam Year 3 data, derives S8 constraints, and tests robustness against systematics and modeling choices.

ABSTRACT

We measure weak lensing cosmic shear power spectra from the three-year galaxy shear catalog of the Hyper Suprime-Cam (HSC) Subaru Strategic Program imaging survey. The shear catalog covers $416 \ \mathrm{deg}^2$ of the northern sky, with a mean $i$-band seeing of 0.59 arcsec and an effective galaxy number density of 15 $\mathrm{arcmin}^{-2}$ within our adopted redshift range. With an $i$-band magnitude limit of 24.5 mag, and four tomographic redshift bins spanning $0.3 \leq z_{\mathrm{ph}} \leq 1.5$ based on photometric redshifts, we obtain a high-significance measurement of the cosmic shear power spectra, with a signal-to-noise ratio of approximately 26.4 in the multipole range $300<\ell<1800$. The accuracy of our power spectrum measurement is tested against realistic mock shear catalogs, and we use these catalogs to get a reliable measurement of the covariance of the power spectrum measurements. We use a robust blinding procedure to avoid confirmation bias, and model various uncertainties and sources of bias in our analysis, including point spread function systematics, redshift distribution uncertainties, the intrinsic alignment of galaxies and the modeling of the matter power spectrum. For a flat $Λ$CDM model, we find $S_8 \equiv σ_8 (Ω_m/0.3)^{0.5} =0.776^{+0.032}_{-0.033}$, which is in excellent agreement with the constraints from the other HSC Year 3 cosmology analyses, as well as those from a number of other cosmic shear experiments. This result implies a $\sim$$2σ$-level tension with the Planck 2018 cosmology. We study the effect that various systematic errors and modeling choices could have on this value, and find that they can shift the best-fit value of $S_8$ by no more than $\sim$$0.5σ$, indicating that our result is robust to such systematics.

Motivation & Objective

• Motivate precise cosmology with weak gravitational lensing via cosmic shear measurements.
• Extract cosmic shear power spectra using tomographic analysis to break parameter degeneracies.
• Model and mitigate systematic uncertainties (PSF, redshift distributions, intrinsic alignments, matter power spectrum).
• Calibrate covariances with realistic mock catalogs and validate through blinding to avoid bias.

Proposed method

• Compute cosmic shear E-mode power spectra in tomographic bins using the Pseudo-C_ell estimator to correct for incomplete sky coverage.
• Decompose the shear field into E and B modes to isolate the cosmological signal and check for systematics.
• Use image-simulated calibrations to correct multiplicative and additive shear biases, including selection biases.
• Estimate covariances from an extensive suite of 1404 mock catalogs based on full-sky lensing simulations.
• Implement a two-level blinding strategy to prevent confirmation bias and unblind only after predefined checks.
• Model and marginalize over systematics such as PSF residuals, redshift distribution uncertainties, intrinsic alignments, and nonlinear matter power spectrum effects.

Experimental results

Research questions

• RQ1 What is the cosmic shear signal level achievable with HSC Year 3 data across 0.3 ≤ z_ph ≤ 1.5 tomographic bins?
• RQ2 What is the inferred S8 parameter in flat ΛCDM from HSC-Y3 cosmic shear power spectra, and how does it compare with Planck and other surveys?
• RQ3 How robust are the S8 constraints to systematic uncertainties and modeling choices (PSF, photo-z, intrinsic alignments, halo modeling)?
• RQ4 Do cross-checks (null tests, B-mode measurements) indicate any significant residual systematics affecting the cosmological interpretation?

Key findings

• S8 = 0.776^{+0.032}_{-0.033} for flat ΛCDM, consistent with other HSC analyses and several cosmic shear surveys.
• High-significance measurement with a signal-to-noise ratio of ~26.4 for 300 < ℓ < 1800.
• No significant B-mode detection, suggesting negligible PSF-related systematics in the measurement.
• Systematics and modeling choices shift S8 by no more than ~0.5σ, indicating robustness of the result.
• Planck 2018 cosmology is in ~2σ tension with the HSC-Y3 S8 constraint.