Cosmic shear cosmology with the UNIONS data

Euclid France meeting, Strasbourg

Sacha Guerrini

CEA Paris-Saclay

November 17, 2025

Weak gravitational lensing

Credit: ESA

• Percent-level effect
• Polluted by shape noise
• Requires a large number of galaxies

The Ultraviolet Near-Infrared Optical Northern Survey (UNIONS, Gwyn et al., 2025)

\(r\), \(u\) bands

\(i\) band

\(g\), \(z\) bands

Euclid photometry in the North

Scaramella+2025

The landscape of lensing surveys

Wright+2025

Technical specifications of UNIONS:

• Target area: \(\sim 5,000\) deg\(^2\)
• Depth: 24.5 (\(r\)-band)
• Seeing: \(\sim 0.69"\) (\(r\)-band)
• Processing with ShapePipe (Farrens et al., 2022)
• \(\sim 100\) million galaxies

Outline of the talk

1. Analysis choices for our cosmic shear analysis
2. \(B\)-modes null test
3. Control of systematics
   • PSF systematics
   • Instrinsic Alignment
4. Blinded constraints

and others…

Analysis choices

	Configuration space (Goh et al., in prep.)	Harmonic space (Guerrini et al., in prep.)
Summary stat.	\(\xi_\pm(\vartheta)\)	\(C_\ell^{EE}\)
Likelihood	Gaussian (CosmoSIS)	Gaussian (CosmoSIS)
Covariance	CosmoCov (analytical)	iNKA + OneCovariance (semi-analytical)
Measurement	TreeCorr	NaMaster
Power spectrum	CAMB + HMCode2020	CAMB + HMCode2020
PSF systematics	Modeled and marginalised	Negligible
Scale cuts	\(B\)-modes test	Theoretical model

\(B\)-modes null test (harmonic space)

Different weak lensing catalog versions:

• SP v1.4.5: Size cut - \(\mathrm{HLR}_\mathrm{gal} / \mathrm{HLR}_\mathrm{PSF} > 0.5\)
• SP v1.4.6: Size cut - \(\mathrm{HLR}_\mathrm{gal} / \mathrm{HLR}_\mathrm{PSF} > 0.7\)
• SP v1.4.8: additionally mask star haloes

\(B\)-modes null test (configuration space)

Credit: C.Daley and A.Guinot

Significant \(B\)-modes on small scales. Scale cuts on large scales derived from PSF leakage estimation.

\(B\)-modes null test (configuration space)

Credit: C.Daley and A.Guinot

Control of systematics

• Astrophysical systematics
  • Intrinsic Alignment
  • Baryons
• Instrumental systematics
  • PSF systematics
  • Shear \(m\)-bias
  • Redshift distribution uncertainty

Control of systematics

• Astrophysical systematics
  • Intrinsic Alignment
  • Baryons
• Instrumental systematics
  • PSF systematics
  • Shear \(m\)-bias
  • Redshift distribution uncertainty

PSF systematics (Guerrini+2025)

PSF error model: \(\delta e^\mathrm{obs} = \underbrace{\alpha e^\mathrm{PSF}}_{\mathrm{leakage}} + \underbrace{\beta (e^* - e^\mathrm{PSF})}_{\textrm{ellipticity residuals}} + \underbrace{\eta e^\mathrm{PSF} \left( \frac{T^* - T^\mathrm{PSF}}{T^*}\right)}_{\textrm{size residuals}}\)

Galaxy-Star correlations: \(\tau_0(\vartheta) = \langle e^\mathrm{obs} e^\mathrm{PSF} \rangle(\vartheta)\), \(\tau_2(\vartheta) = \langle e^\mathrm{obs} \delta e^\mathrm{PSF} \rangle(\vartheta)\), \(\tau_5(\vartheta) = \langle e^\mathrm{obs} \delta T^\mathrm{PSF} \rangle(\vartheta)\)

PSF systematics (Guerrini+2025)

Galaxy-PSF correlation helps to:

• Select our weak lensing sample
• Choose scale cuts for the inference

\[ \xi_\mathrm{sys}(\vartheta) = \alpha \tau_0(\vartheta) + \beta \tau_2(\vartheta) + \eta \tau_5(\vartheta) \]

Intrinsic Alignment

In a non-tomographic setup, \(S_8\) and \(A_\mathrm{IA}\) are strongly degenerate.

\[ A_\mathrm{IA} = f_\mathrm{r} A_\mathrm{IA, r} + f_\mathrm{b} A_\mathrm{IA, b} \]

Hervas Peters+2025

Direct measurements can be used to derive a prior for different galaxy types.

Intrinsic Alignment

1. Separate red and blue galaxies.
2. Estimate red and blue fractions.
3. Estimate \(A_\mathrm{IA, b}\) from direct measurement (Johnston+2019).
4. Estimate \(A_\mathrm{IA, r}\) sampling from IA/luminosity relationship.
5. Add Gaussian errors in quadrature.
6. Doubling the width of the prior to be conservative.

Credit: F. Hervas Peters

Blinded constraints

Analysis blinded by the use of 3 different redshift blinds.

Blinded constraints (harmonic space)

Guerrini+in prep.

Blinded constraints (configuration space)

Goh+in prep.

Conclusion

• First cosmological constraints from UNIONS are incoming.
• We still have to assess the consistency between configuration and harmonic space analyses.
• Some validation tools developed within UNIONS are applied on Euclid data for quality assessment (RR2/TR1/DR1).
• The UNIONS dataset has a unique overlap with spectroscopic surveys (BOSS/eBOSS/DESI).
  • Already publications using galaxy-galaxy lensing on the arXiv.
  • Direct measurement of Intrinsic Alignment (Hervas Peters+2025)
  • 3 publications on Intrinsic Alignment in preparation (see, namely, A. Corinaldi’s talk)

Thank you for your attention and lookout for

Paper index	Author	Title
1	Hervas Peters et al.	UNIONS: Weak lensing catalogues
2	Daley et al.	UNIONS: \(B\)-mode validation and comparison for cosmic shear
3	Hervas Peters et al.	UNIONS: image simulations and validation
4	Goh et al.	UNIONS: Cosmological constraints from cosmic shear in configuration space
5	Guerrini et al.	UNIONS: Cosmological constraints from cosmic shear in harmonic space

Shear \(m\)-bias

\(m\)-bias corrected with MetaCalibration. Further calibration required from image simulations due to blending.

Real exposure

Simulated exposure

Hervas Peters+in prep.

Redshift distribution uncertainty

Analysis blinded by the use of 3 different redshift blinds.

Credit: A.Wittje

PSF systematics (harmonic space)

Scale cuts (harmonic space)

Data vector (harmonic space)

Covariance validation (harmonic space)