UNIONS: A direct measurement of intrinsic alignment with BOSS/eBOSS spectroscopy

• Published in: Astronomy and astrophysics (Berlin) Vol. 699; p. A201
• Main Authors: Hervas Peters, Fabian, Kilbinger, Martin, Paviot, Romain, Baumont, Lucie, Russier, Elisa, Zhang, Ziwen, Murray, Calum, Pettorino, Valeria, de Boer, Thomas, Fabbro, Sébastien, Guerrini, Sacha, Hildebrandt, Hendrik, Hudson, Michael J., Van Waerbeke, Ludovic, Wittje, Anna
• Format: Journal Article
• Language: English
• Published: 01.07.2025
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/202453442

Context. During their formation, galaxies are subject to tidal forces, which create correlations between their shapes and the large-scale structure of the Universe, known as intrinsic alignment. This alignment is a source of contamination for cosmic-shear measurements as we need to disentangle correlations induced by external lensing effects from those intrinsically present in galaxies. Aims. We constrained the amplitude of intrinsic alignment and test models by making use of the overlap between the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS) covering 3500 deg 2 and spectroscopic data from the Baryon Oscillation Spectroscopic Survey (BOSS/eBOSS). By comparing our results to measurements from other lensing surveys on the same spectroscopic tracers, we can test the reliability of these estimates. Methods. We measured projected correlation functions between positions and ellipticities, which we modelled with perturbation theory to constrain the commonly used non-linear alignment model and its higher order expansion. We computed an analytical covariance matrix and validated it using jackknife estimates. Results. Using the non-linear alignment model, we obtained a 13 σ detection with CMASS galaxies, a 3 σ detection with LRGs, and a detection compatible with the null hypothesis for ELGs. We tested the tidal alignment and tidal torque model. This is a higher order alignment model that we found to be in good agreement with the non-linear alignment prediction and for which we were able to constrain the second-order parameters. We demonstrate the strong scaling of our intrinsic alignment amplitude with luminosity. We also demonstrate that the UNIONS sample is robust against systematic contributions, particularly concerning the point spread function (PSF) biases. We reached a reasonable agreement when comparing our measurements to other lensing samples for the same spectroscopic samples. We take this agreement as an indication that direct measurements of intrinsic alignment are mature for stage IV priors.