UNIONS: A direct measurement of intrinsic alignment with BOSS/eBOSS spectroscopy
Bibliographic record
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.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.002 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".