Intrinsic Alignment Workshop

Jun 18, 2026, 10:00 AM → Jun 19, 2026, 5:00 PM Europe/Paris

Salle de l'entre-sol (Institut d'Astrophysique de Paris)

We are organizing a workshop on intrinsic alignments on June 18–19 at the Institut d’Astrophysique de Paris. The event will bring together researchers working on intrinsic alignments in cosmology and astrophysics.

The program will include ample time for in-depth presentations and discussions on a range of topics, including:

• Impact of intrinsic alignments on weak lensing

• Intrinsic alignments within the cosmic web

• Probing large-scale structure with intrinsic alignments

• Current and future observations of intrinsic alignments

 

We warmly welcome contributed talks. For abstract submission, please send us an email before the 20th May. Remote participation and talks are also welcome.

Thu, June 18

10:00 AM Coffee & discussion

1 Review talk astrophysics - Sandrine Codis - 10:30 - 11:30

2 contributed talk - Maëlie Mondelin

Galaxy evolution and alignments in the cosmic web: insights from deep CFHT imaging of the Pisces–Perseus Supercluster

3 contributed talk - Antonin Corinaldi

Title: Inferring the 3D shapes of galaxies inside dark matter halos with UNIONS: preparation for intrinsic alignments studies

Abstract: Galaxies live inside dark matter halos and are subject to intrinsic alignments, which correspond to the correlations between their 3D shapes and 3D orientations and the underlying tidal field of dark matter across the large-scale structures of the Universe. This intrinsic galaxy alignment is traditionally measured with the galaxy-density correlation function projected in the sky plane. However, the projection of 3D shapes introduces an important loss of information about the alignment along the line of sight. In this talk, I will present a novel method to infer the distribution of 3D galaxy shapes from their observed 2D images using simulation-based inference. I use the halo catalogue of the N-body simulation AbacusSummit to extract the 3D shapes and orientations of dark matter halos identified with the CompaSO (Competitive Assignment to Spherical Overdensities) halo finder. Then, these halos are populated by galaxies using a Halo Occupation Distribution (HOD) model. The N-body simulation serves as model for imaging data from the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS). UNIONS is a multi-band optical survey which is going to cover 6250 deg^2 in the Northern sky. I obtain constraints on the distribution of 3D galaxy shapes and on the 3D galaxy-halo connection from 4800 deg^2 of observed sky. In future work, we aim to predict an intrinsic alignment signal using this model of 3D morphology, in order to reveal, additionally to the observed galaxy alignments, extra informations about the connection between the morphology of galaxies and halos in 3D.

12:30 PM Lunch

4 contributed talk - Ankit Singh

Title: Channelled versus Isotropic Accretion onto Clusters: A Spin-Flip and Shape-Alignment Hierarchy from the Cosmic Web

Abstract: We classify ~106,000 galaxy groups around 3,428 clusters in the (1 Gpc/h)^3 MDPL2 simulation by their topological route of infall, along primary filaments (node environment), secondary tributaries (saddle environment), or from the isotropic field, using a complete filament network extracted with a multi-block DisPerSE method (Singh et. al. 2026; under reivew in MNRAS). We report a spin-filament alignment hierarchy: the spin flip mass M_flip = 10^{13.13} for node-feeding filaments and 10^{12.87} for saddle-point tributaries, elevated by 1.2-1.4 dex above the general field, while isotropic groups show no flip. Shape-filament alignment is anti-correlated with spin alignment and increases monotonically with mass, with no shape-flip, consistent with the same tidal field stretching haloes along filaments while torquing their spins perpendicular. These results provide a quantitative test of constrained tidal torque theory predictions for the node-versus-saddle gradient, and demonstrate that intrinsic alignment models that ignore cosmic web topology will conflate physically distinct tidal environments.

5 contributed talk - Clotilde Laigle & Elsa Roy

6 contributed talk - Clotilde Laigle & Elsa Roy

3:30 PM Coffee break

7 Discussion chair by Celine Gouin

Fri, June 19

10:00 AM Coffee & Discussion

8 Review talk cosmology - Romain Paviot - 10:30 -11:30

9 contributed talk Martin Kilbinger

10 contributed talk Aniruddh Herle

Talk title:Intrinsic Alignments in FLAMINGO: Mass, Redshift, and Assembly Bias

Abstract:
Hydrodynamic simulations are becoming an essential tool for informing intrinsic alignment (IA) models for upcoming weak lensing surveys such as Euclid and the Legacy Survey of Space and Time. In this talk, I present new results from the FLAMINGO simulations, using an LRG-like sample of several million galaxies to study the physical drivers of intrinsic alignments and to develop improved models for cosmological analyses.
By jointly modelling galaxy clustering and alignment statistics, we obtain some of the tightest constraints to date on the Non-Linear Alignment (NLA) and Tidal Alignment and Tidal Torquing (TATT) models from a hydrodynamic simulation. We show that both models provide good fits to the data, and introduce a mass-dependent extension of TATT, TATT-M, in which the higher-order parameters are determined empirically as functions of halo mass. This reduces the effective number of free parameters and is very strongly preferred by the data over NLA. TATT-M has been adopted as the fiducial intrinsic alignment model for the Euclid DR1 weak lensing analysis. We also present a new mass- and redshift-dependent model for the intrinsic alignment of Luminous Red Galaxies, calibrated directly on FLAMINGO, designed to capture the evolution of the alignment signal across cosmic time.
Using the feedback variations in FLAMINGO, we show that changes in AGN and supernova feedback do not significantly alter the alignment signal beyond their impact on galaxy stellar mass. We then move beyond the standard assumption that IA depends only on halo mass, demonstrating for the first time that halo and galaxy assembly history also play a key role. Galaxies residing in earlier-forming haloes exhibit systematically stronger alignments, providing clear evidence that assembly bias affects intrinsic alignments. By following galaxies across snapshots, we further show that the redshift evolution of the alignment signal is more complex than assumed in current models.
These results provide new constraints on the physical origin of intrinsic alignments and directly inform model choices and priors for upcoming weak lensing analyses.

12:30 PM Lunch

11 contributed talk - Leonor Simoes

Field-level implementation of the TATT model: projection effects, smoothing choices, and the NLA limit
Intrinsic alignments (IA) are one of the dominant astrophysical systematics for weak lensing surveys, and accurate forward modelling of alignment signals at the field level is essential for next-generation analyses. The tidal alignment and tidal torquing (TATT) model goes beyond the more often used nonlinear alignment (NLA) model by including both linear tidal alignment and quadratic tidal torquing contributions. However, its implementation at the field level in large-volume N-body simulations introduces several modelling choices whose impact is not yet fully understood.
In this work, I present and compare a field-level implementation of the NLA and TATT models applied directly to N-body simulations and investigate how different choices affect the resulting IA signal. In particular, I focus on the impact of the choice of smoothing scale and how smoothing the initial density field changes the relative importance of the tidal alignment and tidal torquing components. I show that the transition between NLA-like and TATT-like behaviour depends sensitively on these choices, affecting both the amplitude and scale dependence of the predicted IA power spectra.
Another challenge is that TATT is formulated in 3D, while observations and even some simulations give us only projected 2D quantities. I discuss how this mismatch affects the consistency between the model and its projected statistics, and present ongoing work towards connecting projected 2D statistics back to the full 3D TATT model.
These results have direct implications for the use of IA models in forward-modelling pipelines and for the interpretation of alignment signals in the context of large-scale structure.

12 contributed talk Calum Murray

13 contributed talk

3:30 PM Coffee break

14 Discussion chair by Calum Murray