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

¹ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
² Dipartimento di Fisica – Sezione di Astronomia, Università di Trieste, Via Tiepolo 11, 34131 Trieste, Italy
³ INAF-Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy
⁴ Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA
⁵ CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei, Anhui 230026, China
⁶ School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China
⁷ European Space Agency/ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
⁸ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁹ NRC Herzberg Astronomy & Astrophysics, 5071 West Saanich Road, British Columbia V9E2E7, Canada
¹⁰ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), German Centre for Cosmological Lensing, 44780 Bochum, Germany
¹¹ Department of Physics and Astronomy, University of Waterloo, Waterloo, ON N2L 3G1, Canada
¹² Waterloo Centre for Astrophysics, University of Waterloo, Waterloo, ON N2L 3G1, Canada
¹³ Perimeter Institute for Theoretical Physics, Waterloo ON N2L 2Y5, Canada
¹⁴ Department of Physics and Astronomy, University of British Columbia, Vancouver, V6T1Z1 BC, Canada

^⋆ Corresponding author: fabian.hervaspeters@cea.fr

Received: 13 December 2024
Accepted: 14 April 2025

Abstract

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 deg² 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 13σ detection with CMASS galaxies, a 3σ 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.